Network Working Group J. Moy
Request for Comments: 2328 Ascend Communications, Inc.
STD: 54 April 1998
Obsoletes: 2178
Category: Standards Track

OSPF Version 2

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is
unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

This memo documents version 2 of the OSPF protocol. OSPF is a
link-state routing protocol. It is designed to be run internal to a
single Autonomous System. Each OSPF router maintains an identical
database describing the Autonomous System's topology. From this
database, a routing table is calculated by constrUCting a shortest-
path tree.

OSPF recalculates routes quickly in the face of topological changes,
utilizing a minimum of routing protocol traffic. OSPF provides
support for equal-cost multipath. An area routing capability is
provided, enabling an additional level of routing protection and a
reduction in routing protocol traffic. In addition, all OSPF
routing protocol exchanges are authenticated.

The differences between this memo and RFC2178 are eXPlained in
Appendix G. All differences are backward-compatible in nature.

Implementations of this memo and of RFCs 2178, 1583, and 1247 will
interoperate.

Please send comments to ospf@gated.cornell.edu.

Table of Contents

1 Introduction ........................................... 6
1.1 Protocol Overview ...................................... 6
1.2 Definitions of commonly used terms ..................... 8
1.3 Brief history of link-state routing technology ........ 11
1.4 Organization of this document ......................... 12
1.5 Acknowledgments ....................................... 12
2 The link-state database: organization and calculations 13
2.1 Representation of routers and networks ................ 13
2.1.1 Representation of non-broadcast networks .............. 15
2.1.2 An example link-state database ........................ 18
2.2 The shortest-path tree ................................ 21
2.3 Use of external routing information ................... 23
2.4 Equal-cost multipath .................................. 26
3 Splitting the AS into Areas ........................... 26
3.1 The backbone of the Autonomous System ................. 27
3.2 Inter-area routing .................................... 27
3.3 Classification of routers ............................. 28
3.4 A sample area configuration ........................... 29
3.5 IP subnetting support ................................. 35
3.6 Supporting stub areas ................................. 37
3.7 Partitions of areas ................................... 38
4 Functional Summary .................................... 40
4.1 Inter-area routing .................................... 41
4.2 AS external routes .................................... 41
4.3 Routing protocol packets .............................. 42
4.4 Basic implementation requirements ..................... 43
4.5 Optional OSPF capabilities ............................ 46
5 Protocol data structures .............................. 47
6 The Area Data Structure ............................... 49
7 Bringing Up Adjacencies ............................... 52
7.1 The Hello Protocol .................................... 52
7.2 The Synchronization of Databases ...................... 53
7.3 The Designated Router ................................. 54
7.4 The Backup Designated Router .......................... 56
7.5 The graph of adjacencies .............................. 56

8 Protocol Packet Processing ............................ 58
8.1 Sending protocol packets .............................. 58
8.2 Receiving protocol packets ............................ 61
9 The Interface Data Structure .......................... 63
9.1 Interface states ...................................... 67
9.2 Events causing interface state changes ................ 70
9.3 The Interface state machine ........................... 72
9.4 Electing the Designated Router ........................ 75
9.5 Sending Hello packets ................................. 77
9.5.1 Sending Hello packets on NBMA networks ................ 79
10 The Neighbor Data Structure ........................... 80
10.1 Neighbor states ....................................... 83
10.2 Events causing neighbor state changes ................. 87
10.3 The Neighbor state machine ............................ 89
10.4 Whether to become adjacent ............................ 95
10.5 Receiving Hello Packets ............................... 96
10.6 Receiving Database Description Packets ................ 99
10.7 Receiving Link State Request Packets ................. 102
10.8 Sending Database Description Packets ................. 103
10.9 Sending Link State Request Packets ................... 104
10.10 An Example ........................................... 105
11 The Routing Table Structure .......................... 107
11.1 Routing table lookup ................................. 111
11.2 Sample routing table, without areas .................. 111
11.3 Sample routing table, with areas ..................... 112
12 Link State Advertisements (LSAs) ..................... 115
12.1 The LSA Header ....................................... 116
12.1.1 LS age ............................................... 116
12.1.2 Options .............................................. 117
12.1.3 LS type .............................................. 117
12.1.4 Link State ID ........................................ 117
12.1.5 Advertising Router ................................... 119
12.1.6 LS sequence number ................................... 120
12.1.7 LS checksum .......................................... 121
12.2 The link state database .............................. 121
12.3 Representation of TOS ................................ 122
12.4 Originating LSAs ..................................... 123
12.4.1 Router-LSAs .......................................... 126
12.4.1.1 Describing point-to-point interfaces ................. 130
12.4.1.2 Describing broadcast and NBMA interfaces ............. 130
12.4.1.3 Describing virtual links ............................. 131
12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 131

12.4.1.5 Examples of router-LSAs .............................. 132
12.4.2 Network-LSAs ......................................... 133
12.4.2.1 Examples of network-LSAs ............................. 134
12.4.3 Summary-LSAs ......................................... 135
12.4.3.1 Originating summary-LSAs into stub areas ............. 137
12.4.3.2 Examples of summary-LSAs ............................. 138
12.4.4 AS-external-LSAs ..................................... 139
12.4.4.1 Examples of AS-external-LSAs ......................... 140
13 The Flooding Procedure ............................... 143
13.1 Determining which LSA is newer ....................... 146
13.2 Installing LSAs in the database ...................... 147
13.3 Next step in the flooding procedure .................. 148
13.4 Receiving self-originated LSAs ....................... 151
13.5 Sending Link State Acknowledgment packets ............ 152
13.6 Retransmitting LSAs .................................. 154
13.7 Receiving link state acknowledgments ................. 155
14 Aging The Link State Database ........................ 156
14.1 Premature aging of LSAs .............................. 157
15 Virtual Links ........................................ 158
16 Calculation of the routing table ..................... 160
16.1 Calculating the shortest-path tree for an area ....... 161
16.1.1 The next hop calculation ............................. 167
16.2 Calculating the inter-area routes .................... 178
16.3 Examining transit areas' summary-LSAs ................ 170
16.4 Calculating AS external routes ....................... 173
16.4.1 External path preferences ............................ 175
16.5 Incremental updates -- summary-LSAs .................. 175
16.6 Incremental updates -- AS-external-LSAs .............. 177
16.7 Events generated as a result of routing table changes 177
16.8 Equal-cost multipath ................................. 178
Footnotes ............................................ 179
References ........................................... 183
A OSPF data formats .................................... 185
A.1 Encapsulation of OSPF packets ........................ 185
A.2 The Options field .................................... 187
A.3 OSPF Packet Formats .................................. 189
A.3.1 The OSPF packet header ............................... 190
A.3.2 The Hello packet ..................................... 193
A.3.3 The Database Description packet ...................... 195
A.3.4 The Link State Request packet ........................ 197
A.3.5 The Link State Update packet ......................... 199
A.3.6 The Link State Acknowledgment packet ................. 201

A.4 LSA formats .......................................... 203
A.4.1 The LSA header ....................................... 204
A.4.2 Router-LSAs .......................................... 206
A.4.3 Network-LSAs ......................................... 210
A.4.4 Summary-LSAs ......................................... 212
A.4.5 AS-external-LSAs ..................................... 214
B Architectural Constants .............................. 217
C Configurable Constants ............................... 219
C.1 Global parameters .................................... 219
C.2 Area parameters ...................................... 220
C.3 Router interface parameters .......................... 221
C.4 Virtual link parameters .............................. 224
C.5 NBMA network parameters .............................. 224
C.6 Point-to-MultiPoint network parameters ............... 225
C.7 Host route parameters ................................ 226
D Authentication ....................................... 227
D.1 Null authentication .................................. 227
D.2 Simple passWord authentication ....................... 228
D.3 Cryptographic authentication ......................... 228
D.4 Message generation ................................... 231
D.4.1 Generating Null authentication ....................... 231
D.4.2 Generating Simple password authentication ............ 232
D.4.3 Generating Cryptographic authentication .............. 232
D.5 Message verification ................................. 234
D.5.1 Verifying Null authentication ........................ 234
D.5.2 Verifying Simple password authentication ............. 234
D.5.3 Verifying Cryptographic authentication ............... 235
E An algorithm for assigning Link State IDs ............ 236
F Multiple interfaces to the same network/subnet ....... 239
G Differences from RFC2178 ............................ 240
G.1 Flooding modifications ............................... 240
G.2 Changes to external path preferences ................. 241
G.3 Incomplete resolution of virtual next hops ........... 241
G.4 Routing table lookup ................................. 241
Security Considerations .............................. 243
Author's Address ..................................... 243
Full Copyright Statement ............................. 244

1. Introduction

This document is a specification of the Open Shortest Path First
(OSPF) TCP/IP internet routing protocol. OSPF is classified as an
Interior Gateway Protocol (IGP). This means that it distributes
routing information between routers belonging to a single Autonomous
System. The OSPF protocol is based on link-state or SPF technology.
This is a departure from the Bellman-Ford base used by traditional
TCP/IP internet routing protocols.

The OSPF protocol was developed by the OSPF working group of the
Internet Engineering Task Force. It has been designed expressly for
the TCP/IP internet environment, including explicit support for CIDR
and the tagging of externally-derived routing information. OSPF
also provides for the authentication of routing updates, and
utilizes IP multicast when sending/receiving the updates. In
addition, much work has been done to produce a protocol that
responds quickly to topology changes, yet involves small amounts of
routing protocol traffic.

1.1. Protocol overview

OSPF routes IP packets based solely on the destination IP
address found in the IP packet header. IP packets are routed
"as is" -- they are not encapsulated in any further protocol
headers as they transit the Autonomous System. OSPF is a
dynamic routing protocol. It quickly detects topological
changes in the AS (such as router interface failures) and
calculates new loop-free routes after a period of convergence.
This period of convergence is short and involves a minimum of
routing traffic.

In a link-state routing protocol, each router maintains a
database describing the Autonomous System's topology. This
database is referred to as the link-state database. Each
participating router has an identical database. Each individual
piece of this database is a particular router's local state
(e.g., the router's usable interfaces and reachable neighbors).
The router distributes its local state throughout the Autonomous
System by flooding.

All routers run the exact same algorithm, in parallel. From the
link-state database, each router constructs a tree of shortest
paths with itself as root. This shortest-path tree gives the
route to each destination in the Autonomous System. Externally
derived routing information appears on the tree as leaves.

When several equal-cost routes to a destination exist, traffic
is distributed equally among them. The cost of a route is
described by a single dimensionless metric.

OSPF allows sets of networks to be grouped together. Such a
grouping is called an area. The topology of an area is hidden
from the rest of the Autonomous System. This information hiding
enables a significant reduction in routing traffic. Also,
routing within the area is determined only by the area's own
topology, lending the area protection from bad routing data. An
area is a generalization of an IP subnetted network.

OSPF enables the flexible configuration of IP subnets. Each
route distributed by OSPF has a destination and mask. Two
different subnets of the same IP network number may have
different sizes (i.e., different masks). This is commonly
referred to as variable length subnetting. A packet is routed
to the best (i.e., longest or most specific) match. Host routes
are considered to be subnets whose masks are "all ones"
(0xffffffff).

All OSPF protocol exchanges are authenticated. This means that
only trusted routers can participate in the Autonomous System's
routing. A variety of authentication schemes can be used; in
fact, separate authentication schemes can be configured for each
IP subnet.

Externally derived routing data (e.g., routes learned from an
Exterior Gateway Protocol such as BGP; see [Ref23]) is
advertised throughout the Autonomous System. This externally
derived data is kept separate from the OSPF protocol's link
state data. Each external route can also be tagged by the
advertising router, enabling the passing of additional
information between routers on the boundary of the Autonomous
System.

1.2. Definitions of commonly used terms

This section provides definitions for terms that have a specific
meaning to the OSPF protocol and that are used throughout the
text. The reader unfamiliar with the Internet Protocol Suite is
referred to [Ref13] for an introduction to IP.

Router
A level three Internet Protocol packet switch. Formerly
called a gateway in much of the IP literature.

Autonomous System
A group of routers exchanging routing information via a
common routing protocol. Abbreviated as AS.

Interior Gateway Protocol
The routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous
System has a single IGP. Separate Autonomous Systems may be
running different IGPs.

Router ID
A 32-bit number assigned to each router running the OSPF
protocol. This number uniquely identifies the router within
an Autonomous System.

Network
In this memo, an IP network/subnet/supernet. It is possible
for one physical network to be assigned multiple IP
network/subnet numbers. We consider these to be separate
networks. Point-to-point physical networks are an exception
- they are considered a single network no matter how many
(if any at all) IP network/subnet numbers are assigned to
them.

Network mask
A 32-bit number indicating the range of IP addresses
residing on a single IP network/subnet/supernet. This
specification displays network masks as hexadecimal numbers.

For example, the network mask for a class C IP network is
displayed as 0xffffff00. Such a mask is often displayed
elsewhere in the literature as 255.255.255.0.

Point-to-point networks
A network that joins a single pair of routers. A 56Kb
serial line is an example of a point-to-point network.

Broadcast networks
Networks supporting many (more than two) attached routers,
together with the capability to address a single physical
message to all of the attached routers (broadcast).
Neighboring routers are discovered dynamically on these nets
using OSPF's Hello Protocol. The Hello Protocol itself
takes advantage of the broadcast capability. The OSPF
protocol makes further use of multicast capabilities, if
they exist. Each pair of routers on a broadcast network is
assumed to be able to communicate directly. An ethernet is
an example of a broadcast network.

Non-broadcast networks
Networks supporting many (more than two) routers, but having
no broadcast capability. Neighboring routers are maintained
on these nets using OSPF's Hello Protocol. However, due to
the lack of broadcast capability, some configuration
information may be necessary to aid in the discovery of
neighbors. On non-broadcast networks, OSPF protocol packets
that are normally multicast need to be sent to each
neighboring router, in turn. An X.25 Public Data Network
(PDN) is an example of a non-broadcast network.

OSPF runs in one of two modes over non-broadcast networks.
The first mode, called non-broadcast multi-Access or NBMA,
simulates the operation of OSPF on a broadcast network. The
second mode, called Point-to-MultiPoint, treats the non-
broadcast network as a collection of point-to-point links.
Non-broadcast networks are referred to as NBMA networks or
Point-to-MultiPoint networks, depending on OSPF's mode of
operation over the network.

Interface
The connection between a router and one of its attached
networks. An interface has state information associated
with it, which is oBTained from the underlying lower level
protocols and the routing protocol itself. An interface to
a network has associated with it a single IP address and
mask (unless the network is an unnumbered point-to-point
network). An interface is sometimes also referred to as a
link.

Neighboring routers
Two routers that have interfaces to a common network.
Neighbor relationships are maintained by, and usually
dynamically discovered by, OSPF's Hello Protocol.

Adjacency
A relationship formed between selected neighboring routers
for the purpose of exchanging routing information. Not
every pair of neighboring routers become adjacent.

Link state advertisement
Unit of data describing the local state of a router or
network. For a router, this includes the state of the
router's interfaces and adjacencies. Each link state
advertisement is flooded throughout the routing domain. The
collected link state advertisements of all routers and
networks forms the protocol's link state database.
Throughout this memo, link state advertisement is
abbreviated as LSA.

Hello Protocol
The part of the OSPF protocol used to establish and maintain
neighbor relationships. On broadcast networks the Hello
Protocol can also dynamically discover neighboring routers.

Flooding
The part of the OSPF protocol that distributes and
synchronizes the link-state database between OSPF routers.

Designated Router
Each broadcast and NBMA network that has at least two
attached routers has a Designated Router. The Designated

Router generates an LSA for the network and has other
special responsibilities in the running of the protocol.
The Designated Router is elected by the Hello Protocol.

The Designated Router concept enables a reduction in the
number of adjacencies required on a broadcast or NBMA
network. This in turn reduces the amount of routing
protocol traffic and the size of the link-state database.

Lower-level protocols
The underlying network access protocols that provide
services to the Internet Protocol and in turn the OSPF
protocol. Examples of these are the X.25 packet and frame
levels for X.25 PDNs, and the ethernet data link layer for
ethernets.

1.3. Brief history of link-state routing technology

OSPF is a link state routing protocol. Such protocols are also
referred to in the literature as SPF-based or distributed-
database protocols. This section gives a brief description of
the developments in link-state technology that have influenced
the OSPF protocol.

The first link-state routing protocol was developed for use in
the ARPANET packet switching network. This protocol is
described in [Ref3]. It has formed the starting point for all
other link-state protocols. The homogeneous ARPANET
environment, i.e., single-vendor packet switches connected by
synchronous serial lines, simplified the design and
implementation of the original protocol.

Modifications to this protocol were proposed in [Ref4]. These
modifications dealt with increasing the fault tolerance of the
routing protocol through, among other things, adding a checksum
to the LSAs (thereby detecting database corruption). The paper
also included means for reducing the routing traffic overhead in
a link-state protocol. This was accomplished by introducing
mechanisms which enabled the interval between LSA originations
to be increased by an order of magnitude.

A link-state algorithm has also been proposed for use as an ISO
IS-IS routing protocol. This protocol is described in [Ref2].
The protocol includes methods for data and routing traffic
reduction when operating over broadcast networks. This is
accomplished by election of a Designated Router for each
broadcast network, which then originates an LSA for the network.

The OSPF Working Group of the IETF has extended this work in
developing the OSPF protocol. The Designated Router concept has
been greatly enhanced to further reduce the amount of routing
traffic required. Multicast capabilities are utilized for
additional routing bandwidth reduction. An area routing scheme
has been developed enabling information
hiding/protection/reduction. Finally, the algorithms have been
tailored for efficient operation in TCP/IP internets.

1.4. Organization of this document

The first three sections of this specification give a general
overview of the protocol's capabilities and functions. Sections
4-16 explain the protocol's mechanisms in detail. Packet
formats, protocol constants and configuration items are
specified in the appendices.

Labels such as HelloInterval encountered in the text refer to
protocol constants. They may or may not be configurable.
Architectural constants are summarized in Appendix B.
Configurable constants are summarized in Appendix C.

The detailed specification of the protocol is presented in terms
of data structures. This is done in order to make the
explanation more precise. Implementations of the protocol are
required to support the functionality described, but need not
use the precise data structures that appear in this memo.

1.5. Acknowledgments

The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
JujJavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui

Zhang and the rest of the OSPF Working Group for the ideas and
support they have given to this project.

The OSPF Point-to-MultiPoint interface is based on work done by
Fred Baker.

The OSPF Cryptographic Authentication option was developed by
Fred Baker and Ran Atkinson.

2. The Link-state Database: organization and calculations

The following subsections describe the organization of OSPF's link-
state database, and the routing calculations that are performed on
the database in order to produce a router's routing table.

2.1. Representation of routers and networks

The Autonomous System's link-state database describes a directed
graph. The vertices of the graph consist of routers and
networks. A graph edge connects two routers when they are
attached via a physical point-to-point network. An edge
connecting a router to a network indicates that the router has
an interface on the network. Networks can be either transit or
stub networks. Transit networks are those capable of carrying
data traffic that is neither locally originated nor locally
destined. A transit network is represented by a graph vertex
having both incoming and outgoing edges. A stub network's vertex
has only incoming edges.

The neighborhood of each network node in the graph depends on
the network's type (point-to-point, broadcast, NBMA or Point-
to-MultiPoint) and the number of routers having an interface to
the network. Three cases are depicted in Figure 1a. Rectangles
indicate routers. Circles and oblongs indicate networks.
Router names are prefixed with the letters RT and network names
with the letter N. Router interface names are prefixed by the
letter I. Lines between routers indicate point-to-point
networks. The left side of the figure shows networks with their
connected routers, with the resulting graphs shown on the right.

**FROM**

* RT1RT2
+---+Ia +---+ * ------------
RT1------RT2 T RT1 X
+---+ Ib+---+ O RT2 X
* Ia X
* Ib X

Physical point-to-point networks

**FROM**
+---+ *
RT7 * RT7 N3
+---+ T ------------
O RT7
+----------------------+ * N3 X
N3 *

Stub networks

**FROM**
+---+ +---+
RT3 RT4 RT3RT4RT5RT6N2
+---+ +---+ * ------------------------
N2 * RT3 X
+----------------------+ T RT4 X
O RT5 X
+---+ +---+ * RT6 X
RT5 RT6 * N2 X X X X
+---+ +---+

Broadcast or NBMA networks

Figure 1a: Network map components

Networks and routers are represented by vertices.
An edge connects Vertex A to Vertex B iff the
intersection of Column A and Row B is marked with
an X.

The top of Figure 1a shows two routers connected by a point-to-
point link. In the resulting link-state database graph, the two
router vertices are directly connected by a pair of edges, one
in each direction. Interfaces to point-to-point networks need
not be assigned IP addresses. When interface addresses are
assigned, they are modelled as stub links, with each router
advertising a stub connection to the other router's interface
address. Optionally, an IP subnet can be assigned to the point-
to-point network. In this case, both routers advertise a stub
link to the IP subnet, instead of advertising each others' IP
interface addresses.

The middle of Figure 1a shows a network with only one attached
router (i.e., a stub network). In this case, the network appears
on the end of a stub connection in the link-state database's
graph.

When multiple routers are attached to a broadcast network, the
link-state database graph shows all routers bidirectionally
connected to the network vertex. This is pictured at the bottom
of Figure 1a.

Each network (stub or transit) in the graph has an IP address
and associated network mask. The mask indicates the number of
nodes on the network. Hosts attached directly to routers
(referred to as host routes) appear on the graph as stub
networks. The network mask for a host route is always
0xffffffff, which indicates the presence of a single node.

2.1.1. Representation of non-broadcast networks

As mentioned previously, OSPF can run over non-broadcast
networks in one of two modes: NBMA or Point-to-MultiPoint.
The choice of mode determines the way that the Hello

protocol and flooding work over the non-broadcast network,
and the way that the network is represented in the link-
state database.

In NBMA mode, OSPF emulates operation over a broadcast
network: a Designated Router is elected for the NBMA
network, and the Designated Router originates an LSA for the
network. The graph representation for broadcast networks and
NBMA networks is identical. This representation is pictured
in the middle of Figure 1a.

NBMA mode is the most efficient way to run OSPF over non-
broadcast networks, both in terms of link-state database
size and in terms of the amount of routing protocol traffic.
However, it has one significant restriction: it requires all
routers attached to the NBMA network to be able to
communicate directly. This restriction may be met on some
non-broadcast networks, such as an ATM subnet utilizing
SVCs. But it is often not met on other non-broadcast
networks, such as PVC-only Frame Relay networks. On non-
broadcast networks where not all routers can communicate
directly you can break the non-broadcast network into
logical subnets, with the routers on each subnet being able
to communicate directly, and then run each separate subnet
as an NBMA network (see [Ref15]). This however requires
quite a bit of administrative overhead, and is prone to
misconfiguration. It is probably better to run such a non-
broadcast network in Point-to-Multipoint mode.

In Point-to-MultiPoint mode, OSPF treats all router-to-
router connections over the non-broadcast network as if they
were point-to-point links. No Designated Router is elected
for the network, nor is there an LSA generated for the
network. In fact, a vertex for the Point-to-MultiPoint
network does not appear in the graph of the link-state
database.

Figure 1b illustrates the link-state database representation
of a Point-to-MultiPoint network. On the left side of the
figure, a Point-to-MultiPoint network is pictured. It is
assumed that all routers can communicate directly, except
for routers RT4 and RT5. I3 though I6 indicate the routers'

IP interface addresses on the Point-to-MultiPoint network.
In the graphical representation of the link-state database,
routers that can communicate directly over the Point-to-
MultiPoint network are joined by bidirectional edges, and
each router also has a stub connection to its own IP
interface address (which is in contrast to the
representation of real point-to-point links; see Figure 1a).

On some non-broadcast networks, use of Point-to-MultiPoint
mode and data-link protocols such as Inverse ARP (see
[Ref14]) will allow autodiscovery of OSPF neighbors even
though broadcast support is not available.

**FROM**
+---+ +---+
RT3 RT4 RT3RT4RT5RT6
+---+ +---+ * --------------------
I3 N2 I4 * RT3 X X X
+----------------------+ T RT4 X X
I5 I6 O RT5 X X
+---+ +---+ * RT6 X X X
RT5 RT6 * I3 X
+---+ +---+ I4 X
I5 X
I6 X

Figure 1b: Network map components
Point-to-MultiPoint networks

All routers can communicate directly over N2, except
routers RT4 and RT5. I3 through I6 indicate IP
interface addresses

2.1.2. An example link-state database

Figure 2 shows a sample map of an Autonomous System. The
rectangle labelled H1 indicates a host, which has a SLIP
connection to Router RT12. Router RT12 is therefore
advertising a host route. Lines between routers indicate
physical point-to-point networks. The only point-to-point
network that has been assigned interface addresses is the
one joining Routers RT6 and RT10. Routers RT5 and RT7 have
BGP connections to other Autonomous Systems. A set of BGP-
learned routes have been displayed for both of these
routers.

A cost is associated with the output side of each router
interface. This cost is configurable by the system
administrator. The lower the cost, the more likely the
interface is to be used to forward data traffic. Costs are
also associated with the externally derived routing data
(e.g., the BGP-learned routes).

The directed graph resulting from the map in Figure 2 is
depicted in Figure 3. Arcs are labelled with the cost of
the corresponding router output interface. Arcs having no
labelled cost have a cost of 0. Note that arcs leading from
networks to routers always have cost 0; they are significant
nonetheless. Note also that the externally derived routing
data appears on the graph as stubs.

The link-state database is pieced together from LSAs
generated by the routers. In the associated graphical
representation, the neighborhood of each router or transit
network is represented in a single, separate LSA. Figure 4
shows these LSAs graphically. Router RT12 has an interface
to two broadcast networks and a SLIP line to a host.
Network N6 is a broadcast network with three attached
routers. The cost of all links from Network N6 to its
attached routers is 0. Note that the LSA for Network N6 is
actually generated by one of the network's attached routers:
the router that has been elected Designated Router for the
network.

+
3+---+ N12 N14
N1--RT1 1 N13 /
+---+ 8 8/8
+ ____ /
/ 1+---+8 8+---+6
* N3 *---RT4------RT5--------+
____/ +---+ +---+
+ / 7
3+---+ /
N2--RT2/1 1 6
+---+ +---+8 6+---+
+ RT3--------------RT6
+---+ +---+
2 Ia7

+---------+
N4


N11
+---------+
N12
3 6 2/
+---+ +---+/
RT9 RT7---N15
+---+ +---+ 9
1 + 1
___ Ib5 ___
/ 1+----+2 3+----+1 /
* N9 *------RT11-------RT10---* N6 *
____/ +----+ +----+ ____/

1 + 1
+--+ 10+----+ N8 +---+
H1-----RT12 RT8
+--+SLIP +----+ +---+
2 4

+---------+ +--------+
N10 N7

Figure 2: A sample Autonomous System

**FROM**

RTRTRTRTRTRTRTRTRTRTRTRT
1 2 3 4 5 6 7 8 9 101112N3N6N8N9
----- ---------------------------------------------
RT1 0
RT2 0
RT3 6 0
RT4 8 0
RT5 8 6 6
RT6 8 7 5
RT7 6 0
* RT8 0
* RT9 0
T RT10 7 0 0
O RT11 0 0
* RT12 0
* N13
N2 3
N31 1 1 1
N4 2
N6 1 1 1
N7 4
N8 3 2
N9 1 1 1
N10 2
N11 3
N12 8 2
N13 8
N14 8
N15 9
H1 10

Figure 3: The resulting directed graph

Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.

**FROM** **FROM**

RT12N9N10H1 RT9RT11RT12N9
* -------------------- * ----------------------
* RT12 * RT9 0
T N91 T RT11 0
O N102 O RT12 0
* H110 * N9
* *
RT12's router-LSA N9's network-LSA

Figure 4: Individual link state components

Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.

2.2. The shortest-path tree

When no OSPF areas are configured, each router in the Autonomous
System has an identical link-state database, leading to an
identical graphical representation. A router generates its
routing table from this graph by calculating a tree of shortest
paths with the router itself as root. Obviously, the shortest-
path tree depends on the router doing the calculation. The
shortest-path tree for Router RT6 in our example is depicted in
Figure 5.

The tree gives the entire path to any destination network or
host. However, only the next hop to the destination is used in
the forwarding process. Note also that the best route to any
router has also been calculated. For the processing of external
data, we note the next hop and distance to any router
advertising external routes. The resulting routing table for
Router RT6 is pictured in Table 2. Note that there is a
separate route for each end of a numbered point-to-point network
(in this case, the serial line between Routers RT6 and RT10).

Routes to networks belonging to other AS'es (such as N12) appear
as dashed lines on the shortest path tree in Figure 5. Use of

RT6(origin)
RT5 o------------o-----------o Ib
/ 6 7
8/88
/ 6
o o 7
N12 o N14
N13 2
N4 o-----o RT3
/ 5
1/ RT10 o-------o Ia
/
RT4 o-----o N3 3 1
/ N6 RT7
/ N8 o o---------o
/ /
RT2 o o RT1 2/ 9
/ RT8 /
/3 3 RT11 o o o o
/ N12 N15
N2 o o N1 1 4

N9 o o N7
/
/
N11 RT9 / RT12
o--------o-------o o--------o H1
3 10
2

o N10

Figure 5: The SPF tree for Router RT6

Edges that are not marked with a cost have a cost of
of zero (these are network-to-router links). Routes
to networks N12-N15 are external information that is
considered in Section 2.3

Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 8

Table 2: The portion of Router RT6's routing table listing local
destinations.

this externally derived routing information is considered in the
next section.

2.3. Use of external routing information

After the tree is created the external routing information is
examined. This external routing information may originate from
another routing protocol such as BGP, or be statically
configured (static routes). Default routes can also be included
as part of the Autonomous System's external routing information.

External routing information is flooded unaltered throughout the
AS. In our example, all the routers in the Autonomous System
know that Router RT7 has two external routes, with metrics 2 and
9.

OSPF supports two types of external metrics. Type 1 external
metrics are expressed in the same units as OSPF interface cost

(i.e., in terms of the link state metric). Type 2 external
metrics are an order of magnitude larger; any Type 2 metric is
considered greater than the cost of any path internal to the AS.
Use of Type 2 external metrics assumes that routing between
AS'es is the major cost of routing a packet, and eliminates the
need for conversion of external costs to internal link state
metrics.

As an example of Type 1 external metric processing, suppose that
the Routers RT7 and RT5 in Figure 2 are advertising Type 1
external metrics. For each advertised external route, the total
cost from Router RT6 is calculated as the sum of the external
route's advertised cost and the distance from Router RT6 to the
advertising router. When two routers are advertising the same
external destination, RT6 picks the advertising router providing
the minimum total cost. RT6 then sets the next hop to the
external destination equal to the next hop that would be used
when routing packets to the chosen advertising router.

In Figure 2, both Router RT5 and RT7 are advertising an external
route to destination Network N12. Router RT7 is preferred since
it is advertising N12 at a distance of 10 (8+2) to Router RT6,
which is better than Router RT5's 14 (6+8). Table 3 shows the
entries that are added to the routing table when external routes
are examined:

Destination Next Hop Distance
__________________________________
N12 RT10 10
N13 RT5 14
N14 RT5 14
N15 RT10 17

Table 3: The portion of Router RT6's routing table
listing external destinations.

Processing of Type 2 external metrics is simpler. The AS
boundary router advertising the smallest external metric is

chosen, regardless of the internal distance to the AS boundary
router. Suppose in our example both Router RT5 and Router RT7
were advertising Type 2 external routes. Then all traffic
destined for Network N12 would be forwarded to Router RT7, since
2 < 8. When several equal-cost Type 2 routes exist, the
internal distance to the advertising routers is used to break
the tie.

Both Type 1 and Type 2 external metrics can be present in the AS
at the same time. In that event, Type 1 external metrics always
take precedence.

This section has assumed that packets destined for external
destinations are always routed through the advertising AS
boundary router. This is not always desirable. For example,
suppose in Figure 2 there is an additional router attached to
Network N6, called Router RTX. Suppose further that RTX does
not participate in OSPF routing, but does exchange BGP
information with the AS boundary router RT7. Then, Router RT7
would end up advertising OSPF external routes for all
destinations that should be routed to RTX. An extra hop will
sometimes be introduced if packets for these destinations need
always be routed first to Router RT7 (the advertising router).

To deal with this situation, the OSPF protocol allows an AS
boundary router to specify a "forwarding address" in its AS-
external-LSAs. In the above example, Router RT7 would specify
RTX's IP address as the "forwarding address" for all those
destinations whose packets should be routed directly to RTX.

The "forwarding address" has one other application. It enables
routers in the Autonomous System's interior to function as
"route servers". For example, in Figure 2 the router RT6 could
become a route server, gaining external routing information
through a combination of static configuration and external
routing protocols. RT6 would then start advertising itself as
an AS boundary router, and would originate a collection of OSPF
AS-external-LSAs. In each AS-external-LSA, Router RT6 would
specify the correct Autonomous System exit point to use for the
destination through appropriate setting of the LSA's "forwarding
address" field.

2.4. Equal-cost multipath

The above discussion has been simplified by considering only a
single route to any destination. In reality, if multiple
equal-cost routes to a destination exist, they are all
discovered and used. This requires no conceptual changes to the
algorithm, and its discussion is postponed until we consider the
tree-building process in more detail.

With equal cost multipath, a router potentially has several
available next hops towards any given destination.

3. Splitting the AS into Areas

OSPF allows collections of contiguous networks and hosts to be
grouped together. Such a group, together with the routers having
interfaces to any one of the included networks, is called an area.
Each area runs a separate copy of the basic link-state routing
algorithm. This means that each area has its own link-state
database and corresponding graph, as explained in the previous
section.

The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
detailed topology external to the area. This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic
as compared to treating the entire Autonomous System as a single
link-state domain.

With the introduction of areas, it is no longer true that all
routers in the AS have an identical link-state database. A router
actually has a separate link-state database for each area it is
connected to. (Routers connected to multiple areas are called area
border routers). Two routers belonging to the same area have, for
that area, identical area link-state databases.

Routing in the Autonomous System takes place on two levels,
depending on whether the source and destination of a packet reside
in the same area (intra-area routing is used) or different areas
(inter-area routing is used). In intra-area routing, the packet is
routed solely on information obtained within the area; no routing

information obtained from outside the area can be used. This
protects intra-area routing from the injection of bad routing
information. We discuss inter-area routing in Section 3.2.

3.1. The backbone of the Autonomous System

The OSPF backbone is the special OSPF Area 0 (often written as
Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP
addresses). The OSPF backbone always contains all area border
routers. The backbone is responsible for distributing routing
information between non-backbone areas. The backbone must be
contiguous. However, it need not be physically contiguous;
backbone connectivity can be established/maintained through the
configuration of virtual links.

Virtual links can be configured between any two backbone routers
that have an interface to a common non-backbone area. Virtual
links belong to the backbone. The protocol treats two routers
joined by a virtual link as if they were connected by an
unnumbered point-to-point backbone network. On the graph of the
backbone, two such routers are joined by arcs whose costs are
the intra-area distances between the two routers. The routing
protocol traffic that flows along the virtual link uses intra-
area routing only.

3.2. Inter-area routing

When routing a packet between two non-backbone areas the
backbone is used. The path that the packet will travel can be
broken up into three contiguous pieces: an intra-area path from
the source to an area border router, a backbone path between the
source and destination areas, and then another intra-area path
to the destination. The algorithm finds the set of such paths
that have the smallest cost.

Looking at this another way, inter-area routing can be pictured
as forcing a star configuration on the Autonomous System, with
the backbone as hub and each of the non-backbone areas as
spokes.

The topology of the backbone dictates the backbone paths used
between areas. The topology of the backbone can be enhanced by
adding virtual links. This gives the system administrator some
control over the routes taken by inter-area traffic.

The correct area border router to use as the packet exits the
source area is chosen in exactly the same way routers
advertising external routes are chosen. Each area border router
in an area summarizes for the area its cost to all networks
external to the area. After the SPF tree is calculated for the
area, routes to all inter-area destinations are calculated by
examining the summaries of the area border routers.

3.3. Classification of routers

Before the introduction of areas, the only OSPF routers having a
specialized function were those advertising external routing
information, such as Router RT5 in Figure 2. When the AS is
split into OSPF areas, the routers are further divided according
to function into the following four overlapping categories:

Internal routers
A router with all directly connected networks belonging to
the same area. These routers run a single copy of the basic
routing algorithm.

Area border routers
A router that attaches to multiple areas. Area border
routers run multiple copies of the basic algorithm, one copy
for each attached area. Area border routers condense the
topological information of their attached areas for
distribution to the backbone. The backbone in turn
distributes the information to the other areas.

Backbone routers
A router that has an interface to the backbone area. This
includes all routers that interface to more than one area
(i.e., area border routers). However, backbone routers do
not have to be area border routers. Routers with all
interfaces connecting to the backbone area are supported.

AS boundary routers
A router that exchanges routing information with routers
belonging to other Autonomous Systems. Such a router
advertises AS external routing information throughout the
Autonomous System. The paths to each AS boundary router are
known by every router in the AS. This classification is
completely independent of the previous classifications: AS
boundary routers may be internal or area border routers, and
may or may not participate in the backbone.

3.4. A sample area configuration

Figure 6 shows a sample area configuration. The first area
consists of networks N1-N4, along with their attached routers
RT1-RT4. The second area consists of networks N6-N8, along with
their attached routers RT7, RT8, RT10 and RT11. The third area
consists of networks N9-N11 and Host H1, along with their
attached routers RT9, RT11 and RT12. The third area has been
configured so that networks N9-N11 and Host H1 will all be
grouped into a single route, when advertised external to the
area (see Section 3.5 for more details).

In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area
border routers. Finally, as before, Routers RT5 and RT7 are AS
boundary routers.

Figure 7 shows the resulting link-state database for the Area 1.
The figure completely describes that area's intra-area routing.
It also shows the complete view of the internet for the two
internal routers RT1 and RT2. It is the job of the area border
routers, RT3 and RT4, to advertise into Area 1 the distances to
all destinations external to the area. These are indicated in
Figure 7 by the dashed stub routes. Also, RT3 and RT4 must
advertise into Area 1 the location of the AS boundary routers
RT5 and RT7. Finally, AS-external-LSAs from RT5 and RT7 are
flooded throughout the entire AS, and in particular throughout
Area 1. These LSAs are included in Area 1's database, and yield
routes to Networks N12-N15.

Routers RT3 and RT4 must also summarize Area 1's topology for

...........................
. + .
. 3+---+ . N12 N14
. N1--RT1 1 . N13 /
. +---+ . 8 8/8
. + ____ . /
. / 1+---+8 8+---+6
. * N3 *---RT4------RT5--------+
. ____/ +---+ +---+
. + / . 7
. 3+---+ / .
. N2--RT2/1 1 . 6
. +---+ +---+8 6+---+
. + RT3------RT6
. +---+ +---+
. 2/ . Ia7
. / .
. +---------+ .
.Area 1 N4 .
...........................
..........................
. N11 .
. +---------+ .
. . N12
. 3 . Ib5 6 2/
. +---+ . +----+ +---+/
. RT9 . .........RT10.....RT7---N15.
. +---+ . . +----+ +---+ 9 .
. 1 . . + /3 1 1 .
. ___ . . / ___ .
. / 1+----+2 / / .
. * N9 *------RT11---- * N6 * .
. ____/ +----+ ____/ .
. . . .
. 1 . . + 1 .
. +--+ 10+----+ . . N8 +---+ .
. H1-----RT12 . . RT8 .
. +--+SLIP +----+ . . +---+ .
. 2 . . 4 .
. . . .
. +---------+ . . +--------+ .

. N10 . . N7 .
. . .Area 2 .
.Area 3 . ................................
..........................

Figure 6: A sample OSPF area configuration

distribution to the backbone. Their backbone LSAs are shown in
Table 4. These summaries show which networks are contained in
Area 1 (i.e., Networks N1-N4), and the distance to these
networks from the routers RT3 and RT4 respectively.

The link-state database for the backbone is shown in Figure 8.
The set of routers pictured are the backbone routers. Router
RT11 is a backbone router because it belongs to two areas. In
order to make the backbone connected, a virtual link has been
configured between Routers R10 and R11.

The area border routers RT3, RT4, RT7, RT10 and RT11 condense
the routing information of their attached non-backbone areas for
distribution via the backbone; these are the dashed stubs that
appear in Figure 8. Remember that the third area has been
configured to condense Networks N9-N11 and Host H1 into a single
route. This yields a single dashed line for networks N9-N11 and
Host H1 in Figure 8. Routers RT5 and RT7 are AS boundary
routers; their externally derived information also appears on
the graph in Figure 8 as stubs.

Network RT3 adv. RT4 adv.
_____________________________
N1 4 4
N2 4 4
N3 1 1
N4 2 3

Table 4: Networks advertised to the backbone
by Routers RT3 and RT4.

**FROM**

RTRTRTRTRTRT
1 2 3 4 5 7 N3
----- -------------------
RT1 0
RT2 0
RT3 0
* RT4 0
* RT5 148
T RT7 2014
O N13
* N2 3
* N31 1 1 1
N4 2
Ia,Ib 2027
N6 1615
N7 2019
N8 1818
N9-N11,H1 2936
N12 8 2
N13 8
N14 8
N15 9

Figure 7: Area 1's Database.

Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.

**FROM**

RTRTRTRTRTRTRT
3 4 5 6 7 1011
------------------------
RT3 6
RT4 8
RT5 8 6 6
RT68 7 5
RT7 6
* RT10 7 2
* RT11 3
T N14 4
O N24 4
* N31 1
* N42 3
Ia 5
Ib 7
N6 1 1 3
N7 5 5 7
N8 4 3 2
N9-N11,H1 11
N12 8 2
N13 8
N14 8
N15 9

Figure 8: The backbone's database.

Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.

The backbone enables the exchange of summary information between
area border routers. Every area border router hears the area
summaries from all other area border routers. It then forms a
picture of the distance to all networks outside of its area by
examining the collected LSAs, and adding in the backbone
distance to each advertising router.

Again using Routers RT3 and RT4 as an example, the procedure
goes as follows: They first calculate the SPF tree for the
backbone. This gives the distances to all other area border
routers. Also noted are the distances to networks (Ia and Ib)
and AS boundary routers (RT5 and RT7) that belong to the
backbone. This calculation is shown in Table 5.

Next, by looking at the area summaries from these area border
routers, RT3 and RT4 can determine the distance to all networks
outside their area. These distances are then advertised
internally to the area by RT3 and RT4. The advertisements that
Router RT3 and RT4 will make into Area 1 are shown in Table 6.
Note that Table 6 assumes that an area range has been configured
for the backbone which groups Ia and Ib into a single LSA.

The information imported into Area 1 by Routers RT3 and RT4
enables an internal router, such as RT1, to choose an area
border router intelligently. Router RT1 would use RT4 for
traffic to Network N6, RT3 for traffic to Network N10, and would

dist from dist from
RT3 RT4
__________________________________
to RT3 * 21
to RT4 22 *
to RT7 20 14
to RT10 15 22
to RT11 18 25
__________________________________
to Ia 20 27
to Ib 15 22
__________________________________
to RT5 14 8
to RT7 20 14

Table 5: Backbone distances calculated
by Routers RT3 and RT4.

Destination RT3 adv. RT4 adv.
_________________________________
Ia,Ib 20 27
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 29 36
_________________________________
RT5 14 8
RT7 20 14

Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4.

load share between the two for traffic to Network N8.

Router RT1 can also determine in this manner the shortest path
to the AS boundary routers RT5 and RT7. Then, by looking at RT5
and RT7's AS-external-LSAs, Router RT1 can decide between RT5 or
RT7 when sending to a destination in another Autonomous System
(one of the networks N12-N15).

Note that a failure of the line between Routers RT6 and RT10
will cause the backbone to become disconnected. Configuring a
virtual link between Routers RT7 and RT10 will give the backbone
more connectivity and more resistance to such failures.

3.5. IP subnetting support

OSPF attaches an IP address mask to each advertised route. The
mask indicates the range of addresses being described by the
particular route. For example, a summary-LSA for the
destination 128.185.0.0 with a mask of 0xffff0000 actually is
describing a single route to the collection of destinations
128.185.0.0 - 128.185.255.255. Similarly, host routes are
always advertised with a mask of 0xffffffff, indicating the
presence of only a single destination.

Including the mask with each advertised destination enables the
implementation of what is commonly referred to as variable-
length subnetting. This means that a single IP class A, B, or C
network number can be broken up into many subnets of various
sizes. For example, the network 128.185.0.0 could be broken up
into 62 variable-sized subnets: 15 subnets of size 4K, 15
subnets of size 256, and 32 subnets of size 8. Table 7 shows
some of the resulting network addresses together with their
masks.

Network address IP address mask Subnet size
_______________________________________________
128.185.16.0 0xfffff000 4K
128.185.1.0 0xffffff00 256
128.185.0.8 0xfffffff8 8

Table 7: Some sample subnet sizes.

There are many possible ways of dividing up a class A, B, and C
network into variable sized subnets. The precise procedure for
doing so is beyond the scope of this specification. This
specification however establishes the following guideline: When
an IP packet is forwarded, it is always forwarded to the network
that is the best match for the packet's destination. Here best
match is synonymous with the longest or most specific match.
For example, the default route with destination of 0.0.0.0 and
mask 0x00000000 is always a match for every IP destination. Yet
it is always less specific than any other match. Subnet masks
must be assigned so that the best match for any IP destination
is unambiguous.

Attaching an address mask to each route also enables the support
of IP supernetting. For example, a single physical network
segment could be assigned the [address,mask] pair
[192.9.4.0,0xfffffc00]. The segment would then be single IP
network, containing addresses from the four consecutive class C
network numbers 192.9.4.0 through 192.9.7.0. Such addressing is
now becoming commonplace with the advent of CIDR (see [Ref10]).

In order to get better aggregation at area boundaries, area
address ranges can be employed (see Section C.2 for more
details). Each address range is defined as an [address,mask]
pair. Many separate networks may then be contained in a single
address range, just as a subnetted network is composed of many
separate subnets. Area border routers then summarize the area
contents (for distribution to the backbone) by advertising a
single route for each address range. The cost of the route is
the maximum cost to any of the networks falling in the specified
range.

For example, an IP subnetted network might be configured as a
single OSPF area. In that case, a single address range could be
configured: a class A, B, or C network number along with its
natural IP mask. Inside the area, any number of variable sized
subnets could be defined. However, external to the area a
single route for the entire subnetted network would be
distributed, hiding even the fact that the network is subnetted
at all. The cost of this route is the maximum of the set of
costs to the component subnets.

3.6. Supporting stub areas

In some Autonomous Systems, the majority of the link-state
database may consist of AS-external-LSAs. An OSPF AS-external-
LSA is usually flooded throughout the entire AS. However, OSPF
allows certain areas to be configured as "stub areas". AS-
external-LSAs are not flooded into/throughout stub areas;
routing to AS external destinations in these areas is based on a
(per-area) default only. This reduces the link-state database
size, and therefore the memory requirements, for a stub area's
internal routers.

In order to take advantage of the OSPF stub area support,
default routing must be used in the stub area. This is
accomplished as follows. One or more of the stub area's area
border routers must advertise a default route into the stub area
via summary-LSAs. These summary defaults are flooded throughout
the stub area, but no further. (For this reason these defaults
pertain only to the particular stub area). These summary
default routes will be used for any destination that is not

explicitly reachable by an intra-area or inter-area path (i.e.,
AS external destinations).

An area can be configured as a stub when there is a single exit
point from the area, or when the choice of exit point need not
be made on a per-external-destination basis. For example, Area
3 in Figure 6 could be configured as a stub area, because all
external traffic must travel though its single area border
router RT11. If Area 3 were configured as a stub, Router RT11
would advertise a default route for distribution inside Area 3
(in a summary-LSA), instead of flooding the AS-external-LSAs for
Networks N12-N15 into/throughout the area.

The OSPF protocol ensures that all routers belonging to an area
agree on whether the area has been configured as a stub. This
guarantees that no confusion will arise in the flooding of AS-
external-LSAs.

There are a couple of restrictions on the use of stub areas.
Virtual links cannot be configured through stub areas. In
addition, AS boundary routers cannot be placed internal to stub
areas.

3.7. Partitions of areas

OSPF does not actively attempt to repair area partitions. When
an area becomes partitioned, each component simply becomes a
separate area. The backbone then performs routing between the
new areas. Some destinations reachable via intra-area routing
before the partition will now require inter-area routing.

However, in order to maintain full routing after the partition,
an address range must not be split across multiple components of
the area partition. Also, the backbone itself must not
partition. If it does, parts of the Autonomous System will
become unreachable. Backbone partitions can be repaired by
configuring virtual links (see Section 15).

Another way to think about area partitions is to look at the
Autonomous System graph that was introduced in Section 2. Area
IDs can be viewed as colors for the graph's edges.[1] Each edge

of the graph connects to a network, or is itself a point-to-
point network. In either case, the edge is colored with the
network's Area ID.

A group of edges, all having the same color, and interconnected
by vertices, represents an area. If the topology of the
Autonomous System is intact, the graph will have several regions
of color, each color being a distinct Area ID.

When the AS topology changes, one of the areas may become
partitioned. The graph of the AS will then have multiple
regions of the same color (Area ID). The routing in the
Autonomous System will continue to function as long as these
regions of same color are connected by the single backbone
region.

4. Functional Summary

A separate copy of OSPF's basic routing algorithm runs in each area.
Routers having interfaces to multiple areas run multiple copies of
the algorithm. A brief summary of the routing algorithm follows.

When a router starts, it first initializes the routing protocol data
structures. The router then waits for indications from the lower-
level protocols that its interfaces are functional.

A router then uses the OSPF's Hello Protocol to acquire neighbors.
The router sends Hello packets to its neighbors, and in turn
receives their Hello packets. On broadcast and point-to-point
networks, the router dynamically detects its neighboring routers by
sending its Hello packets to the multicast address AllSPFRouters.
On non-broadcast networks, some configuration information may be
necessary in order to discover neighbors. On broadcast and NBMA
networks the Hello Protocol also elects a Designated router for the
network.

The router will attempt to form adjacencies with some of its newly
acquired neighbors. Link-state databases are synchronized between
pairs of adjacent routers. On broadcast and NBMA networks, the
Designated Router determines which routers should become adjacent.

Adjacencies control the distribution of routing information.
Routing updates are sent and received only on adjacencies.

A router periodically advertises its state, which is also called
link state. Link state is also advertised when a router's state
changes. A router's adjacencies are reflected in the contents of
its LSAs. This relationship between adjacencies and link state
allows the protocol to detect dead routers in a timely fashion.

LSAs are flooded throughout the area. The flooding algorithm is
reliable, ensuring that all routers in an area have exactly the same
link-state database. This database consists of the collection of
LSAs originated by each router belonging to the area. From this
database each router calculates a shortest-path tree, with itself as
root. This shortest-path tree in turn yields a routing table for
the protocol.

4.1. Inter-area routing

The previous section described the operation of the protocol
within a single area. For intra-area routing, no other routing
information is pertinent. In order to be able to route to
destinations outside of the area, the area border routers inject
additional routing information into the area. This additional
information is a distillation of the rest of the Autonomous
System's topology.

This distillation is accomplished as follows: Each area border
router is by definition connected to the backbone. Each area
border router summarizes the topology of its attached non-
backbone areas for transmission on the backbone, and hence to
all other area border routers. An area border router then has
complete topological information concerning the backbone, and
the area summaries from each of the other area border routers.
From this information, the router calculates paths to all
inter-area destinations. The router then advertises these paths
into its attached areas. This enables the area's internal
routers to pick the best exit router when forwarding traffic
inter-area destinations.

4.2. AS external routes

Routers that have information regarding other Autonomous Systems
can flood this information throughout the AS. This external
routing information is distributed verbatim to every
participating router. There is one exception: external routing
information is not flooded into "stub" areas (see Section 3.6).

To utilize external routing information, the path to all routers
advertising external information must be known throughout the AS
(excepting the stub areas). For that reason, the locations of
these AS boundary routers are summarized by the (non-stub) area
border routers.

4.3. Routing protocol packets

The OSPF protocol runs directly over IP, using IP protocol 89.
OSPF does not provide any explicit fragmentation/reassembly
support. When fragmentation is necessary, IP
fragmentation/reassembly is used. OSPF protocol packets have
been designed so that large protocol packets can generally be
split into several smaller protocol packets. This practice is
recommended; IP fragmentation should be avoided whenever
possible.

Routing protocol packets should always be sent with the IP TOS
field set to 0. If at all possible, routing protocol packets
should be given preference over regular IP data traffic, both
when being sent and received. As an aid to accomplishing this,
OSPF protocol packets should have their IP precedence field set
to the value Internetwork Control (see [Ref5]).

All OSPF protocol packets share a common protocol header that is
described in Appendix A. The OSPF packet types are listed below
in Table 8. Their formats are also described in Appendix A.

Type Packet name Protocol function
__________________________________________________________
1 Hello Discover/maintain neighbors
2 Database Description Summarize database contents
3 Link State Request Database download
4 Link State Update Database update
5 Link State Ack Flooding acknowledgment

Table 8: OSPF packet types.

OSPF's Hello protocol uses Hello packets to discover and
maintain neighbor relationships. The Database Description and
Link State Request packets are used in the forming of
adjacencies. OSPF's reliable update mechanism is implemented by
the Link State Update and Link State Acknowledgment packets.

Each Link State Update packet carries a set of new link state
advertisements (LSAs) one hop further away from their point of
origination. A single Link State Update packet may contain the
LSAs of several routers. Each LSA is tagged with the ID of the
originating router and a checksum of its link state contents.
Each LSA also has a type field; the different types of OSPF LSAs
are listed below in Table 9.

OSPF routing packets (with the exception of Hellos) are sent
only over adjacencies. This means that all OSPF protocol
packets travel a single IP hop, except those that are sent over
virtual adjacencies. The IP source address of an OSPF protocol
packet is one end of a router adjacency, and the IP destination
address is either the other end of the adjacency or an IP
multicast address.

4.4. Basic implementation requirements

An implementation of OSPF requires the following pieces of
system support:

Timers
Two different kind of timers are required. The first kind,
called "single shot timers", fire once and cause a protocol
event to be processed. The second kind, called "interval
timers", fire at continuous intervals. These are used for
the sending of packets at regular intervals. A good example
of this is the regular broadcast of Hello packets. The
granularity of both kinds of timers is one second.

Interval timers should be implemented to avoid drift. In
some router implementations, packet processing can affect
timer execution. When multiple routers are attached to a
single network, all doing broadcasts, this can lead to the
synchronization of routing packets (which should be
avoided). If timers cannot be implemented to avoid drift,
small random amounts should be added to/subtracted from the
interval timer at each firing.

LS LSA LSA description
type name
________________________________________________________
1 Router-LSAs Originated by all routers.
This LSA describes
the collected states of the
router's interfaces to an
area. Flooded throughout a
single area only.
________________________________________________________
2 Network-LSAs Originated for broadcast
and NBMA networks by
the Designated Router. This
LSA contains the
list of routers connected
to the network. Flooded
throughout a single area only.
________________________________________________________
3,4 Summary-LSAs Originated by area border
routers, and flooded through-
out the LSA's associated
area. Each summary-LSA
describes a route to a
destination outside the area,
yet still inside the AS
(i.e., an inter-area route).
Type 3 summary-LSAs describe
routes to networks. Type 4
summary-LSAs describe
routes to AS boundary routers.
________________________________________________________
5 AS-external-LSAs Originated by AS boundary
routers, and flooded through-
out the AS. Each
AS-external-LSA describes
a route to a destination in
another Autonomous System.
Default routes for the AS can
also be described by
AS-external-LSAs.

Table 9: OSPF link state advertisements (LSAs).

IP multicast
Certain OSPF packets take the form of IP multicast
datagrams. Support for receiving and sending IP multicast
datagrams, along with the appropriate lower-level protocol
support, is required. The IP multicast datagrams used by
OSPF never travel more than one hop. For this reason, the
ability to forward IP multicast datagrams is not required.
For information on IP multicast, see [Ref7].

Variable-length subnet support
The router's IP protocol support must include the ability to
divide a single IP class A, B, or C network number into many
subnets of various sizes. This is commonly called
variable-length subnetting; see Section 3.5 for details.

IP supernetting support
The router's IP protocol support must include the ability to
aggregate contiguous collections of IP class A, B, and C
networks into larger quantities called supernets.
Supernetting has been proposed as one way to improve the
scaling of IP routing in the worldwide Internet. For more
information on IP supernetting, see [Ref10].

Lower-level protocol support
The lower level protocols referred to here are the network
access protocols, such as the Ethernet data link layer.
Indications must be passed from these protocols to OSPF as
the network interface goes up and down. For example, on an
ethernet it would be valuable to know when the ethernet
transceiver cable becomes unplugged.

Non-broadcast lower-level protocol support
On non-broadcast networks, the OSPF Hello Protocol can be
aided by providing an indication when an attempt is made to
send a packet to a dead or non-existent router. For
example, on an X.25 PDN a dead neighboring router may be

indicated by the reception of a X.25 clear with an
appropriate cause and diagnostic, and this information would
be passed to OSPF.

List manipulation primitives
Much of the OSPF functionality is described in terms of its
operation on lists of LSAs. For example, the collection of
LSAs that will be retransmitted to an adjacent router until
acknowledged are described as a list. Any particular LSA
may be on many such lists. An OSPF implementation needs to
be able to manipulate these lists, adding and deleting
constituent LSAs as necessary.

TaSKINg support
Certain procedures described in this specification invoke
other procedures. At times, these other procedures should
be executed in-line, that is, before the current procedure
is finished. This is indicated in the text by instructions
to execute a procedure. At other times, the other
procedures are to be executed only when the current
procedure has finished. This is indicated by instructions
to schedule a task.

4.5. Optional OSPF capabilities

The OSPF protocol defines several optional capabilities. A
router indicates the optional capabilities that it supports in
its OSPF Hello packets, Database Description packets and in its
LSAs. This enables routers supporting a mix of optional
capabilities to coexist in a single Autonomous System.

Some capabilities must be supported by all routers attached to a
specific area. In this case, a router will not accept a
neighbor's Hello Packet unless there is a match in reported
capabilities (i.e., a capability mismatch prevents a neighbor
relationship from forming). An example of this is the
ExternalRoutingCapability (see below).

Other capabilities can be negotiated during the Database
Exchange process. This is accomplished by specifying the
optional capabilities in Database Description packets. A

capability mismatch with a neighbor in this case will result in
only a subset of the link state database being exchanged between
the two neighbors.

The routing table build process can also be affected by the
presence/absence of optional capabilities. For example, since
the optional capabilities are reported in LSAs, routers
incapable of certain functions can be avoided when building the
shortest path tree.

The OSPF optional capabilities defined in this memo are listed
below. See Section A.2 for more information.

ExternalRoutingCapability
Entire OSPF areas can be configured as "stubs" (see Section
3.6). AS-external-LSAs will not be flooded into stub areas.
This capability is represented by the E-bit in the OSPF
Options field (see Section A.2). In order to ensure
consistent configuration of stub areas, all routers
interfacing to such an area must have the E-bit clear in
their Hello packets (see Sections 9.5 and 10.5).

5. Protocol Data Structures

The OSPF protocol is described herein in terms of its operation on
various protocol data structures. The following list comprises the
top-level OSPF data structures. Any initialization that needs to be
done is noted. OSPF areas, interfaces and neighbors also have
associated data structures that are described later in this
specification.

Router ID
A 32-bit number that uniquely identifies this router in the AS.
One possible implementation strategy would be to use the
smallest IP interface address belonging to the router. If a
router's OSPF Router ID is changed, the router's OSPF software
should be restarted before the new Router ID takes effect. In
this case the router should flush its self-originated LSAs from
the routing domain (see Section 14.1) before restarting, or they
will persist for up to MaxAge minutes.

Area structures
Each one of the areas to which the router is connected has its
own data structure. This data structure describes the working
of the basic OSPF algorithm. Remember that each area runs a
separate copy of the basic OSPF algorithm.

Backbone (area) structure
The OSPF backbone area is responsible for the dissemination of
inter-area routing information.

Virtual links configured
The virtual links configured with this router as one endpoint.
In order to have configured virtual links, the router itself
must be an area border router. Virtual links are identified by
the Router ID of the other endpoint -- which is another area
border router. These two endpoint routers must be attached to a
common area, called the virtual link's Transit area. Virtual
links are part of the backbone, and behave as if they were
unnumbered point-to-point networks between the two routers. A
virtual link uses the intra-area routing of its Transit area to
forward packets. Virtual links are brought up and down through
the building of the shortest-path trees for the Transit area.

List of external routes
These are routes to destinations external to the Autonomous
System, that have been gained either through direct experience
with another routing protocol (such as BGP), or through
configuration information, or through a combination of the two
(e.g., dynamic external information to be advertised by OSPF
with configured metric). Any router having these external routes
is called an AS boundary router. These routes are advertised by
the router into the OSPF routing domain via AS-external-LSAs.

List of AS-external-LSAs
Part of the link-state database. These have originated from the
AS boundary routers. They comprise routes to destinations
external to the Autonomous System. Note that, if the router is
itself an AS boundary router, some of these AS-external-LSAs
have been self-originated.

The routing table
Derived from the link-state database. Each entry in the routing
table is indexed by a destination, and contains the
destination's cost and a set of paths to use in forwarding
packets to the destination. A path is described by its type and
next hop. For more information, see Section 11.

Figure 9 shows the collection of data structures present in a
typical router. The router pictured is RT10, from the map in Figure
6. Note that Router RT10 has a virtual link configured to Router
RT11, with Area 2 as the link's Transit area. This is indicated by
the dashed line in Figure 9. When the virtual link becomes active,
through the building of the shortest path tree for Area 2, it
becomes an interface to the backbone (see the two backbone
interfaces depicted in Figure 9).

6. The Area Data Structure

The area data structure contains all the information used to run the
basic OSPF routing algorithm. Each area maintains its own link-state
database. A network belongs to a single area, and a router interface
connects to a single area. Each router adjacency also belongs to a
single area.

The OSPF backbone is the special OSPF area responsible for
disseminating inter-area routing information.

The area link-state database consists of the collection of router-
LSAs, network-LSAs and summary-LSAs that have originated from the
area's routers. This information is flooded throughout a single
area only. The list of AS-external-LSAs (see Section 5) is also
considered to be part of each area's link-state database.

Area ID
A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
reserved for the backbone.

List of area address ranges
In order to aggregate routing information at area boundaries,
area address ranges can be employed. Each address range is
specified by an [address,mask] pair and a status indication of
either Advertise or DoNotAdvertise (see Section 12.4.3).

+----+
RT10------+
+----+ +-------------+
/ Routing Table
/ +-------------+
/
+------+ / +--------+
Area 2---+ +---Backbone
+------+***********+ +--------+
/ * /
/ * /
+---------+ +---------+ +------------+ +------------+
Interface Interface Virtual Link Interface Ib
to N6 to N8 to RT11 +------------+
+---------+ +---------+ +------------+
/
/
+--------+ +--------+ +-------------+ +------------+
Neighbor Neighbor Neighbor RT11 Neighbor RT6
RT8 RT7 +-------------+ +------------+
+--------+ +--------+

+-------------+
Neighbor RT11
+-------------+

Figure 9: Router RT10's Data structures

Associated router interfaces
This router's interfaces connecting to the area. A router
interface belongs to one and only one area (or the backbone).
For the backbone area this list includes all the virtual links.
A virtual link is identified by the Router ID of its other
endpoint; its cost is the cost of the shortest intra-area path
through the Transit area that exists between the two routers.

List of router-LSAs
A router-LSA is generated by each router in the area. It
describes the state of the router's interfaces to the area.

List of network-LSAs
One network-LSA is generated for each transit broadcast and NBMA
network in the area. A network-LSA describes the set of routers
currently connected to the network.

List of summary-LSAs
Summary-LSAs originate from the area's area border routers.
They describe routes to destinations internal to the Autonomous
System, yet external to the area (i.e., inter-area
destinations).

Shortest-path tree
The shortest-path tree for the area, with this router itself as
root. Derived from the collected router-LSAs and network-LSAs
by the Dijkstra algorithm (see Section 16.1).

TransitCapability
This parameter indicates whether the area can carry data traffic
that neither originates nor terminates in the area itself. This
parameter is calculated when the area's shortest-path tree is
built (see Section 16.1, where TransitCapability is set to TRUE
if and only if there are one or more fully adjacent virtual
links using the area as Transit area), and is used as an input
to a subsequent step of the routing table build process (see
Section 16.3). When an area's TransitCapability is set to TRUE,
the area is said to be a "transit area".

ExternalRoutingCapability
Whether AS-external-LSAs will be flooded into/throughout the
area. This is a configurable parameter. If AS-external-LSAs
are excluded from the area, the area is called a "stub". Within
stub areas, routing to AS external destinations will be based
solely on a default summary route. The backbone cannot be
configured as a stub area. Also, virtual links cannot be
configured through stub areas. For more information, see
Section 3.6.

StubDefaultCost
If the area has been configured as a stub area, and the router
itself is an area border router, then the StubDefaultCost
indicates the cost of the default summary-LSA that the router
should advertise into the area. See Section 12.4.3 for more
information.

Unless otherwise specified, the remaining sections of this document
refer to the operation of the OSPF protocol within a single area.

7. Bringing Up Adjacencies

OSPF creates adjacencies between neighboring routers for the purpose
of exchanging routing information. Not every two neighboring
routers will become adjacent. This section covers the generalities
involved in creating adjacencies. For further details consult
Section 10.

7.1. The Hello Protocol

The Hello Protocol is responsible for establishing and
maintaining neighbor relationships. It also ensures that
communication between neighbors is bidirectional. Hello packets
are sent periodically out all router interfaces. Bidirectional
communication is indicated when the router sees itself listed in
the neighbor's Hello Packet. On broadcast and NBMA networks,
the Hello Protocol elects a Designated Router for the network.

The Hello Protocol works differently on broadcast networks, NBMA
networks and Point-to-MultiPoint networks. On broadcast
networks, each router advertises itself by periodically
multicasting Hello Packets. This allows neighbors to be
discovered dynamically. These Hello Packets contain the
router's view of the Designated Router's identity, and the list
of routers whose Hello Packets have been seen recently.

On NBMA networks some configuration information may be necessary
for the operation of the Hello Protocol. Each router that may
potentially become Designated Router has a list of all other

routers attached to the network. A router, having Designated
Router potential, sends Hello Packets to all other potential
Designated Routers when its interface to the NBMA network first
becomes operational. This is an attempt to find the Designated
Router for the network. If the router itself is elected
Designated Router, it begins sending Hello Packets to all other
routers attached to the network.

On Point-to-MultiPoint networks, a router sends Hello Packets to
all neighbors with which it can communicate directly. These
neighbors may be discovered dynamically through a protocol such
as Inverse ARP (see [Ref14]), or they may be configured.

After a neighbor has been discovered, bidirectional
communication ensured, and (if on a broadcast or NBMA network) a
Designated Router elected, a decision is made regarding whether
or not an adjacency should be formed with the neighbor (see
Section 10.4). If an adjacency is to be formed, the first step
is to synchronize the neighbors' link-state databases. This is
covered in the next section.

7.2. The Synchronization of Databases

In a link-state routing algorithm, it is very important for all
routers' link-state databases to stay synchronized. OSPF
simplifies this by requiring only adjacent routers to remain
synchronized. The synchronization process begins as soon as the
routers attempt to bring up the adjacency. Each router
describes its database by sending a sequence of Database
Description packets to its neighbor. Each Database Description
Packet describes a set of LSAs belonging to the router's
database. When the neighbor sees an LSA that is more recent
than its own database copy, it makes a note that this newer LSA
should be requested.

This sending and receiving of Database Description packets is
called the "Database Exchange Process". During this process,
the two routers form a master/slave relationship. Each Database
Description Packet has a sequence number. Database Description
Packets sent by the master (polls) are acknowledged by the slave
through echoing of the sequence number. Both polls and their

responses contain summaries of link state data. The master is
the only one allowed to retransmit Database Description Packets.
It does so only at fixed intervals, the length of which is the
configured per-interface constant RxmtInterval.

Each Database Description contains an indication that there are
more packets to follow --- the M-bit. The Database Exchange
Process is over when a router has received and sent Database
Description Packets with the M-bit off.

During and after the Database Exchange Process, each router has
a list of those LSAs for which the neighbor has more up-to-date
instances. These LSAs are requested in Link State Request
Packets. Link State Request packets that are not satisfied are
retransmitted at fixed intervals of time RxmtInterval. When the
Database Description Process has completed and all Link State
Requests have been satisfied, the databases are deemed
synchronized and the routers are marked fully adjacent. At this
time the adjacency is fully functional and is advertised in the
two routers' router-LSAs.

The adjacency is used by the flooding procedure as soon as the
Database Exchange Process begins. This simplifies database
synchronization, and guarantees that it finishes in a
predictable period of time.

7.3. The Designated Router

Every broadcast and NBMA network has a Designated Router. The
Designated Router performs two main functions for the routing
protocol:

o The Designated Router originates a network-LSA on behalf of
the network. This LSA lists the set of routers (including
the Designated Router itself) currently attached to the
network. The Link State ID for this LSA (see Section
12.1.4) is the IP interface address of the Designated
Router. The IP network number can then be obtained by using
the network's subnet/network mask.

o The Designated Router becomes adjacent to all other routers
on the network. Since the link state databases are
synchronized across adjacencies (through adjacency bring-up
and then the flooding procedure), the Designated Router
plays a central part in the synchronization process.

The Designated Router is elected by the Hello Protocol. A
router's Hello Packet contains its Router Priority, which is
configurable on a per-interface basis. In general, when a
router's interface to a network first becomes functional, it
checks to see whether there is currently a Designated Router for
the network. If there is, it accepts that Designated Router,
regardless of its Router Priority. (This makes it harder to
predict the identity of the Designated Router, but ensures that
the Designated Router changes less often. See below.)
Otherwise, the router itself becomes Designated Router if it has
the highest Router Priority on the network. A more detailed
(and more accurate) description of Designated Router election is
presented in Section 9.4.

The Designated Router is the endpoint of many adjacencies. In
order to optimize the flooding procedure on broadcast networks,
the Designated Router multicasts its Link State Update Packets
to the address AllSPFRouters, rather than sending separate
packets over each adjacency.

Section 2 of this document discusses the directed graph
representation of an area. Router nodes are labelled with their
Router ID. Transit network nodes are actually labelled with the
IP address of their Designated Router. It follows that when the
Designated Router changes, it appears as if the network node on
the graph is replaced by an entirely new node. This will cause
the network and all its attached routers to originate new LSAs.
Until the link-state databases again converge, some temporary
loss of connectivity may result. This may result in ICMP
unreachable messages being sent in response to data traffic.
For that reason, the Designated Router should change only
infrequently. Router Priorities should be configured so that
the most dependable router on a network eventually becomes
Designated Router.

7.4. The Backup Designated Router

In order to make the transition to a new Designated Router
smoother, there is a Backup Designated Router for each broadcast
and NBMA network. The Backup Designated Router is also adjacent
to all routers on the network, and becomes Designated Router
when the previous Designated Router fails. If there were no
Backup Designated Router, when a new Designated Router became
necessary, new adjacencies would have to be formed between the
new Designated Router and all other routers attached to the
network. Part of the adjacency forming process is the
synchronizing of link-state databases, which can potentially
take quite a long time. During this time, the network would not
be available for transit data traffic. The Backup Designated
obviates the need to form these adjacencies, since they already
exist. This means the period of disruption in transit traffic
lasts only as long as it takes to flood the new LSAs (which
announce the new Designated Router).

The Backup Designated Router does not generate a network-LSA for
the network. (If it did, the transition to a new Designated
Router would be even faster. However, this is a tradeoff
between database size and speed of convergence when the
Designated Router disappears.)

The Backup Designated Router is also elected by the Hello
Protocol. Each Hello Packet has a field that specifies the
Backup Designated Router for the network.

In some steps of the flooding procedure, the Backup Designated
Router plays a passive role, letting the Designated Router do
more of the work. This cuts down on the amount of local routing
traffic. See Section 13.3 for more information.

7.5. The graph of adjacencies

An adjacency is bound to the network that the two routers have
in common. If two routers have multiple networks in common,
they may have multiple adjacencies between them.

One can picture the collection of adjacencies on a network as
forming an undirected graph. The vertices consist of routers,
with an edge joining two routers if they are adjacent. The
graph of adjacencies describes the flow of routing protocol
packets, and in particular Link State Update Packets, through
the Autonomous System.

Two graphs are possible, depending on whether a Designated
Router is elected for the network. On physical point-to-point
networks, Point-to-MultiPoint networks and virtual links,
neighboring routers become adjacent whenever they can
communicate directly. In contrast, on broadcast and NBMA
networks only the Designated Router and the Backup Designated
Router become adjacent to all other routers attached to the
network.

+---+ +---+
RT1------------RT2 o---------------o
+---+ N1 +---+ RT1 RT2

RT7
o---------+
+---+ +---+ +---+ /
RT7 RT3 RT4 /
+---+ +---+ +---+ /
/
+-----------------------+ RT5o RT6o oRT4
N2 * * *
+---+ +---+ * * *
RT5 RT6 * * *
+---+ +---+ ***
o---------+
RT3

Figure 10: The graph of adjacencies

These graphs are shown in Figure 10. It is assumed that Router
RT7 has become the Designated Router, and Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated
Router performs a lesser function during the flooding procedure
than the Designated Router (see Section 13.3). This is the
reason for the dashed lines connecting the Backup Designated
Router RT3.

8. Protocol Packet Processing

This section discusses the general processing of OSPF routing
protocol packets. It is very important that the router link-state
databases remain synchronized. For this reason, routing protocol
packets should get preferential treatment over ordinary data
packets, both in sending and receiving.

Routing protocol packets are sent along adjacencies only (with the
exception of Hello packets, which are used to discover the
adjacencies). This means that all routing protocol packets travel a
single IP hop, except those sent over virtual links.

All routing protocol packets begin with a standard header. The
sections below provide details on how to fill in and verify this
standard header. Then, for each packet type, the section giving
more details on that particular packet type's processing is listed.

8.1. Sending protocol packets

When a router sends a routing protocol packet, it fills in the
fields of the standard OSPF packet header as follows. For more
details on the header format consult Section A.3.1:

Version #
Set to 2, the version number of the protocol as documented
in this specification.

Packet type
The type of OSPF packet, such as Link state Update or Hello
Packet.

Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.

Router ID
The identity of the router itself (who is originating the
packet).

Area ID
The OSPF area that the packet is being sent into.

Checksum
The standard IP 16-bit one's complement checksum of the
entire OSPF packet, excluding the 64-bit authentication
field. This checksum is calculated as part of the
appropriate authentication procedure; for some OSPF
authentication types, the checksum calculation is omitted.
See Section D.4 for details.

AuType and Authentication
Each OSPF packet exchange is authenticated. Authentication
types are assigned by the protocol and are documented in
Appendix D. A different authentication procedure can be
used for each IP network/subnet. Autype indicates the type
of authentication procedure in use. The 64-bit
authentication field is then for use by the chosen
authentication procedure. This procedure should be the last
called when forming the packet to be sent. See Section D.4
for details.

The IP destination address for the packet is selected as
follows. On physical point-to-point networks, the IP
destination is always set to the address AllSPFRouters. On all
other network types (including virtual links), the majority of
OSPF packets are sent as unicasts, i.e., sent directly to the
other end of the adjacency. In this case, the IP destination is
just the Neighbor IP address associated with the other end of
the adjacency (see Section 10). The only packets not sent as
unicasts are on broadcast networks; on these networks Hello
packets are sent to the multicast destination AllSPFRouters, the
Designated Router and its Backup send both Link State Update

Packets and Link State Acknowledgment Packets to the multicast
address AllSPFRouters, while all other routers send both their
Link State Update and Link State Acknowledgment Packets to the
multicast address AllDRouters.

Retransmissions of Link State Update packets are ALWAYS sent
directly to the neighbor. On multi-access networks, this means
that retransmissions should be sent to the neighbor's IP
address.

The IP source address should be set to the IP address of the
sending interface. Interfaces to unnumbered point-to-point
networks have no associated IP address. On these interfaces,
the IP source should be set to any of the other IP addresses
belonging to the router. For this reason, there must be at
least one IP address assigned to the router.[2] Note that, for
most purposes, virtual links act precisely the same as
unnumbered point-to-point networks. However, each virtual link
does have an IP interface address (discovered during the routing
table build process) which is used as the IP source when sending
packets over the virtual link.

For more information on the format of specific OSPF packet
types, consult the sections listed in Table 10.

Type Packet name detailed section (transmit)
_________________________________________________________
1 Hello Section 9.5
2 Database description Section 10.8
3 Link state request Section 10.9
4 Link state update Section 13.3
5 Link state ack Section 13.5

Table 10: Sections describing OSPF protocol packet transmission.

8.2. Receiving protocol packets

Whenever a protocol packet is received by the router it is
marked with the interface it was received on. For routers that
have virtual links configured, it may not be immediately obvious
which interface to associate the packet with. For example,
consider the Router RT11 depicted in Figure 6. If RT11 receives
an OSPF protocol packet on its interface to Network N8, it may
want to associate the packet with the interface to Area 2, or
with the virtual link to Router RT10 (which is part of the
backbone). In the following, we assume that the packet is
initially associated with the non-virtual link.[3]

In order for the packet to be accepted at the IP level, it must
pass a number of tests, even before the packet is passed to OSPF
for processing:

o The IP checksum must be correct.

o The packet's IP destination address must be the IP address
of the receiving interface, or one of the IP multicast
addresses AllSPFRouters or AllDRouters.

o The IP protocol specified must be OSPF (89).

o Locally originated packets should not be passed on to OSPF.
That is, the source IP address should be examined to make
sure this is not a multicast packet that the router itself
generated.

Next, the OSPF packet header is verified. The fields specified
in the header must match those configured for the receiving
interface. If they do not, the packet should be discarded:

o The version number field must specify protocol version 2.

o The Area ID found in the OSPF header must be verified. If
both of the following cases fail, the packet should be
discarded. The Area ID specified in the header must either:

(1) Match the Area ID of the receiving interface. In this
case, the packet has been sent over a single hop.
Therefore, the packet's IP source address is required to
be on the same network as the receiving interface. This
can be verified by comparing the packet's IP source
address to the interface's IP address, after masking
both addresses with the interface mask. This comparison
should not be performed on point-to-point networks. On
point-to-point networks, the interface addresses of each
end of the link are assigned independently, if they are
assigned at all.

(2) Indicate the backbone. In this case, the packet has
been sent over a virtual link. The receiving router
must be an area border router, and the Router ID
specified in the packet (the source router) must be the
other end of a configured virtual link. The receiving
interface must also attach to the virtual link's
configured Transit area. If all of these checks
succeed, the packet is accepted and is from now on
associated with the virtual link (and the backbone
area).

o Packets whose IP destination is AllDRouters should only be
accepted if the state of the receiving interface is DR or
Backup (see Section 9.1).

o The AuType specified in the packet must match the AuType
specified for the associated area.

o The packet must be authenticated. The authentication
procedure is indicated by the setting of AuType (see
Appendix D). The authentication procedure may use one or
more Authentication keys, which can be configured on a per-
interface basis. The authentication procedure may also
verify the checksum field in the OSPF packet header (which,
when used, is set to the standard IP 16-bit one's complement
checksum of the OSPF packet's contents after excluding the
64-bit authentication field). If the authentication
procedure fails, the packet should be discarded.

If the packet type is Hello, it should then be further processed
by the Hello Protocol (see Section 10.5). All other packet
types are sent/received only on adjacencies. This means that
the packet must have been sent by one of the router's active
neighbors. If the receiving interface connects to a broadcast
network, Point-to-MultiPoint network or NBMA network the sender
is identified by the IP source address found in the packet's IP
header. If the receiving interface connects to a point-to-point
network or a virtual link, the sender is identified by the
Router ID (source router) found in the packet's OSPF header.
The data structure associated with the receiving interface
contains the list of active neighbors. Packets not matching any
active neighbor are discarded.

At this point all received protocol packets are associated with
an active neighbor. For the further input processing of
specific packet types, consult the sections listed in Table 11.

Type Packet name detailed section (receive)
________________________________________________________
1 Hello Section 10.5
2 Database description Section 10.6
3 Link state request Section 10.7
4 Link state update Section 13
5 Link state ack Section 13.7

Table 11: Sections describing OSPF protocol packet reception.

9. The Interface Data Structure

An OSPF interface is the connection between a router and a network.
We assume a single OSPF interface to each attached network/subnet,
although supporting multiple interfaces on a single network is
considered in Appendix F. Each interface structure has at most one
IP interface address.

An OSPF interface can be considered to belong to the area that
contains the attached network. All routing protocol packets
originated by the router over this interface are labelled with the
interface's Area ID. One or more router adjacencies may develop
over an interface. A router's LSAs reflect the state of its
interfaces and their associated adjacencies.

The following data items are associated with an interface. Note
that a number of these items are actually configuration for the
attached network; such items must be the same for all routers
connected to the network.

Type
The OSPF interface type is either point-to-point, broadcast,
NBMA, Point-to-MultiPoint or virtual link.

State
The functional level of an interface. State determines whether
or not full adjacencies are allowed to form over the interface.
State is also reflected in the router's LSAs.

IP interface address
The IP address associated with the interface. This appears as
the IP source address in all routing protocol packets originated
over this interface. Interfaces to unnumbered point-to-point
networks do not have an associated IP address.

IP interface mask
Also referred to as the subnet mask, this indicates the portion
of the IP interface address that identifies the attached
network. Masking the IP interface address with the IP interface
mask yields the IP network number of the attached network. On
point-to-point networks and virtual links, the IP interface mask
is not defined. On these networks, the link itself is not
assigned an IP network number, and so the addresses of each side
of the link are assigned independently, if they are assigned at
all.

Area ID
The Area ID of the area to which the attached network belongs.
All routing protocol packets originating from the interface are
labelled with this Area ID.

HelloInterval
The length of time, in seconds, between the Hello packets that
the router sends on the interface. Advertised in Hello packets
sent out this interface.

RouterDeadInterval
The number of seconds before the router's neighbors will declare
it down, when they stop hearing the router's Hello Packets.
Advertised in Hello packets sent out this interface.

InfTransDelay
The estimated number of seconds it takes to transmit a Link
State Update Packet over this interface. LSAs contained in the
Link State Update packet will have their age incremented by this
amount before transmission. This value should take into account
transmission and propagation delays; it must be greater than
zero.

Router Priority
An 8-bit unsigned integer. When two routers attached to a
network both attempt to become Designated Router, the one with
the highest Router Priority takes precedence. A router whose
Router Priority is set to 0 is ineligible to become Designated
Router on the attached network. Advertised in Hello packets
sent out this interface.

Hello Timer
An interval timer that causes the interface to send a Hello
packet. This timer fires every HelloInterval seconds. Note
that on non-broadcast networks a separate Hello packet is sent
to each qualified neighbor.

Wait Timer
A single shot timer that causes the interface to exit the
Waiting state, and as a consequence select a Designated Router
on the network. The length of the timer is RouterDeadInterval
seconds.

List of neighboring routers
The other routers attached to this network. This list is formed
by the Hello Protocol. Adjacencies will be formed to some of

these neighbors. The set of adjacent neighbors can be
determined by an examination of all of the neighbors' states.

Designated Router
The Designated Router selected for the attached network. The
Designated Router is selected on all broadcast and NBMA networks
by the Hello Protocol. Two pieces of identification are kept
for the Designated Router: its Router ID and its IP interface
address on the network. The Designated Router advertises link
state for the network; this network-LSA is labelled with the
Designated Router's IP address. The Designated Router is
initialized to 0.0.0.0, which indicates the lack of a Designated
Router.

Backup Designated Router
The Backup Designated Router is also selected on all broadcast
and NBMA networks by the Hello Protocol. All routers on the
attached network become adjacent to both the Designated Router
and the Backup Designated Router. The Backup Designated Router
becomes Designated Router when the current Designated Router
fails. The Backup Designated Router is initialized to 0.0.0.0,
indicating the lack of a Backup Designated Router.

Interface output cost(s)
The cost of sending a data packet on the interface, expressed in
the link state metric. This is advertised as the link cost for
this interface in the router-LSA. The cost of an interface must
be greater than zero.

RxmtInterval
The number of seconds between LSA retransmissions, for
adjacencies belonging to this interface. Also used when
retransmitting Database Description and Link State Request
Packets.

AuType
The type of authentication used on the attached network/subnet.
Authentication types are defined in Appendix D. All OSPF packet
exchanges are authenticated. Different authentication schemes
may be used on different networks/subnets.

Authentication key
This configured data allows the authentication procedure to
generate and/or verify OSPF protocol packets. The
Authentication key can be configured on a per-interface basis.
For example, if the AuType indicates simple password, the
Authentication key would be a 64-bit clear password which is
inserted into the OSPF packet header. If instead Autype
indicates Cryptographic authentication, then the Authentication
key is a shared secret which enables the generation/verification
of message digests which are appended to the OSPF protocol
packets. When Cryptographic authentication is used, multiple
simultaneous keys are supported in order to achieve smooth key
transition (see Section D.3).

9.1. Interface states

The various states that router interfaces may attain is
documented in this section. The states are listed in order of
progressing functionality. For example, the inoperative state
is listed first, followed by a list of intermediate states
before the final, fully functional state is achieved. The
specification makes use of this ordering by sometimes making
references such as "those interfaces in state greater than X".
Figure 11 shows the graph of interface state changes. The arcs
of the graph are labelled with the event causing the state
change. These events are documented in Section 9.2. The
interface state machine is described in more detail in Section
9.3.

Down
This is the initial interface state. In this state, the
lower-level protocols have indicated that the interface is
unusable. No protocol traffic at all will be sent or
received on such a interface. In this state, interface
parameters should be set to their initial values. All
interface timers should be disabled, and there should be no
adjacencies associated with the interface.

Loopback
In this state, the router's interface to the network is

+----+ UnloopInd +--------+
Down<--------------Loopback
+----+ +--------+

InterfaceUp
+-------+ +--------------+
Waiting<-+-------------->Point-to-point
+-------+ +--------------+

WaitTimerBackupSeen


NeighborChange
+------+ +-+<---------------- +-------+
Backup<----------?----------------->DROther
+------+---------->+-+<-----+ +-------+
Neighbor
Change Neighbor
Change
+--+
+---->DR
+--+

Figure 11: Interface State changes

In addition to the state transitions pictured,
Event InterfaceDown always forces Down State, and
Event LoopInd always forces Loopback State

looped back. The interface may be looped back in hardware
or software. The interface will be unavailable for regular
data traffic. However, it may still be desirable to gain
information on the quality of this interface, either through
sending ICMP pings to the interface or through something
like a bit error test. For this reason, IP packets may
still be addressed to an interface in Loopback state. To

facilitate this, such interfaces are advertised in router-
LSAs as single host routes, whose destination is the IP
interface address.[4]

Waiting
In this state, the router is trying to determine the
identity of the (Backup) Designated Router for the network.
To do this, the router monitors the Hello Packets it
receives. The router is not allowed to elect a Backup
Designated Router nor a Designated Router until it
transitions out of Waiting state. This prevents unnecessary
changes of (Backup) Designated Router.

Point-to-point
In this state, the interface is operational, and connects
either to a physical point-to-point network or to a virtual
link. Upon entering this state, the router attempts to form
an adjacency with the neighboring router. Hello Packets are
sent to the neighbor every HelloInterval seconds.

DR Other
The interface is to a broadcast or NBMA network on which
another router has been selected to be the Designated
Router. In this state, the router itself has not been
selected Backup Designated Router either. The router forms
adjacencies to both the Designated Router and the Backup
Designated Router (if they exist).

Backup
In this state, the router itself is the Backup Designated
Router on the attached network. It will be promoted to
Designated Router when the present Designated Router fails.
The router establishes adjacencies to all other routers
attached to the network. The Backup Designated Router
performs slightly different functions during the Flooding
Procedure, as compared to the Designated Router (see Section
13.3). See Section 7.4 for more details on the functions
performed by the Backup Designated Router.

DR In this state, this router itself is the Designated Router
on the attached network. Adjacencies are established to all
other routers attached to the network. The router must also

originate a network-LSA for the network node. The network-
LSA will contain links to all routers (including the
Designated Router itself) attached to the network. See
Section 7.3 for more details on the functions performed by
the Designated Router.

9.2. Events causing interface state changes

State changes can be effected by a number of events. These
events are pictured as the labelled arcs in Figure 11. The
label definitions are listed below. For a detailed explanation
of the effect of these events on OSPF protocol operation,
consult Section 9.3.

InterfaceUp
Lower-level protocols have indicated that the network
interface is operational. This enables the interface to
transition out of Down state. On virtual links, the
interface operational indication is actually a result of the
shortest path calculation (see Section 16.7).

WaitTimer
The Wait Timer has fired, indicating the end of the waiting
period that is required before electing a (Backup)
Designated Router.

BackupSeen
The router has detected the existence or non-existence of a
Backup Designated Router for the network. This is done in
one of two ways. First, an Hello Packet may be received
from a neighbor claiming to be itself the Backup Designated
Router. Alternatively, an Hello Packet may be received from
a neighbor claiming to be itself the Designated Router, and
indicating that there is no Backup Designated Router. In
either case there must be bidirectional communication with
the neighbor, i.e., the router must also appear in the
neighbor's Hello Packet. This event signals an end to the
Waiting state.

NeighborChange
There has been a change in the set of bidirectional
neighbors associated with the interface. The (Backup)
Designated Router needs to be recalculated. The following
neighbor changes lead to the NeighborChange event. For an
explanation of neighbor states, see Section 10.1.

o Bidirectional communication has been established to a
neighbor. In other words, the state of the neighbor has
transitioned to 2-Way or higher.

o There is no longer bidirectional communication with a
neighbor. In other words, the state of the neighbor has
transitioned to Init or lower.

o One of the bidirectional neighbors is newly declaring
itself as either Designated Router or Backup Designated
Router. This is detected through examination of that
neighbor's Hello Packets.

o One of the bidirectional neighbors is no longer
declaring itself as Designated Router, or is no longer
declaring itself as Backup Designated Router. This is
again detected through examination of that neighbor's
Hello Packets.

o The advertised Router Priority for a bidirectional
neighbor has changed. This is again detected through
examination of that neighbor's Hello Packets.

LoopInd
An indication has been received that the interface is now
looped back to itself. This indication can be received
either from network management or from the lower level
protocols.

UnloopInd
An indication has been received that the interface is no
longer looped back. As with the LoopInd event, this

indication can be received either from network management or
from the lower level protocols.

InterfaceDown
Lower-level protocols indicate that this interface is no
longer functional. No matter what the current interface
state is, the new interface state will be Down.

9.3. The Interface state machine

A detailed description of the interface state changes follows.
Each state change is invoked by an event (Section 9.2). This
event may produce different effects, depending on the current
state of the interface. For this reason, the state machine
below is organized by current interface state and received
event. Each entry in the state machine describes the resulting
new interface state and the required set of additional actions.

When an interface's state changes, it may be necessary to
originate a new router-LSA. See Section 12.4 for more details.

Some of the required actions below involve generating events for
the neighbor state machine. For example, when an interface
becomes inoperative, all neighbor connections associated with
the interface must be destroyed. For more information on the
neighbor state machine, see Section 10.3.

State(s): Down

Event: InterfaceUp

New state: Depends upon action routine

Action: Start the interval Hello Timer, enabling the
periodic sending of Hello packets out the interface.
If the attached network is a physical point-to-point
network, Point-to-MultiPoint network or virtual
link, the interface state transitions to Point-to-
Point. Else, if the router is not eligible to
become Designated Router the interface state
transitions to DR Other.

Otherwise, the attached network is a broadcast or
NBMA network and the router is eligible to become
Designated Router. In this case, in an attempt to
discover the attached network's Designated Router
the interface state is set to Waiting and the single
shot Wait Timer is started. Additionally, if the
network is an NBMA network examine the configured
list of neighbors for this interface and generate
the neighbor event Start for each neighbor that is
also eligible to become Designated Router.

State(s): Waiting

Event: BackupSeen

New state: Depends upon action routine.

Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.

State(s): Waiting

Event: WaitTimer

New state: Depends upon action routine.

Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.

State(s): DR Other, Backup or DR

Event: NeighborChange

New state: Depends upon action routine.

Action: Recalculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.

State(s): Any State

Event: InterfaceDown

New state: Down

Action: All interface variables are reset, and interface
timers disabled. Also, all neighbor connections
associated with the interface are destroyed. This
is done by generating the event KillNbr on all
associated neighbors (see Section 10.2).

State(s): Any State

Event: LoopInd

New state: Loopback

Action: Since this interface is no longer connected to the
attached network the actions associated with the
above InterfaceDown event are executed.

State(s): Loopback

Event: UnloopInd

New state: Down

Action: No actions are necessary. For example, the
interface variables have already been reset upon
entering the Loopback state. Note that reception of

an InterfaceUp event is necessary before the
interface again becomes fully functional.

9.4. Electing the Designated Router

This section describes the algorithm used for calculating a
network's Designated Router and Backup Designated Router. This
algorithm is invoked by the Interface state machine. The
initial time a router runs the election algorithm for a network,
the network's Designated Router and Backup Designated Router are
initialized to 0.0.0.0. This indicates the lack of both a
Designated Router and a Backup Designated Router.

The Designated Router election algorithm proceeds as follows:
Call the router doing the calculation Router X. The list of
neighbors attached to the network and having established
bidirectional communication with Router X is examined. This
list is precisely the collection of Router X's neighbors (on
this network) whose state is greater than or equal to 2-Way (see
Section 10.1). Router X itself is also considered to be on the
list. Discard all routers from the list that are ineligible to
become Designated Router. (Routers having Router Priority of 0
are ineligible to become Designated Router.) The following
steps are then executed, considering only those routers that
remain on the list:

(1) Note the current values for the network's Designated Router
and Backup Designated Router. This is used later for
comparison purposes.

(2) Calculate the new Backup Designated Router for the network
as follows. Only those routers on the list that have not
declared themselves to be Designated Router are eligible to
become Backup Designated Router. If one or more of these
routers have declared themselves Backup Designated Router
(i.e., they are currently listing themselves as Backup
Designated Router, but not as Designated Router, in their
Hello Packets) the one having highest Router Priority is
declared to be Backup Designated Router. In case of a tie,
the one having the highest Router ID is chosen. If no
routers have declared themselves Backup Designated Router,

choose the router having highest Router Priority, (again
excluding those routers who have declared themselves
Designated Router), and again use the Router ID to break
ties.

(3) Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared
themselves Designated Router (i.e., they are currently
listing themselves as Designated Router in their Hello
Packets) the one having highest Router Priority is declared
to be Designated Router. In case of a tie, the one having
the highest Router ID is chosen. If no routers have
declared themselves Designated Router, assign the Designated
Router to be the same as the newly elected Backup Designated
Router.

(4) If Router X is now newly the Designated Router or newly the
Backup Designated Router, or is now no longer the Designated
Router or no longer the Backup Designated Router, repeat
steps 2 and 3, and then proceed to step 5. For example, if
Router X is now the Designated Router, when step 2 is
repeated X will no longer be eligible for Backup Designated
Router election. Among other things, this will ensure that
no router will declare itself both Backup Designated Router
and Designated Router.[5]

(5) As a result of these calculations, the router itself may now
be Designated Router or Backup Designated Router. See
Sections 7.3 and 7.4 for the additional duties this would
entail. The router's interface state should be set
accordingly. If the router itself is now Designated Router,
the new interface state is DR. If the router itself is now
Backup Designated Router, the new interface state is Backup.
Otherwise, the new interface state is DR Other.

(6) If the attached network is an NBMA network, and the router
itself has just become either Designated Router or Backup
Designated Router, it must start sending Hello Packets to
those neighbors that are not eligible to become Designated
Router (see Section 9.5.1). This is done by invoking the
neighbor event Start for each neighbor having a Router
Priority of 0.

(7) If the above calculations have caused the identity of either
the Designated Router or Backup Designated Router to change,
the set of adjacencies associated with this interface will
need to be modified. Some adjacencies may need to be
formed, and others may need to be broken. To accomplish
this, invoke the event AdjOK? on all neighbors whose state
is at least 2-Way. This will cause their eligibility for
adjacency to be reexamined (see Sections 10.3 and 10.4).

The reason behind the election algorithm's complexity is the
desire for an orderly transition from Backup Designated Router
to Designated Router, when the current Designated Router fails.
This orderly transition is ensured through the introduction of
hysteresis: no new Backup Designated Router can be chosen until
the old Backup accepts its new Designated Router
responsibilities.

The above procedure may elect the same router to be both
Designated Router and Backup Designated Router, although that
router will never be the calculating router (Router X) itself.
The elected Designated Router may not be the router having the
highest Router Priority, nor will the Backup Designated Router
necessarily have the second highest Router Priority. If Router
X is not itself eligible to become Designated Router, it is
possible that neither a Backup Designated Router nor a
Designated Router will be selected in the above procedure. Note
also that if Router X is the only attached router that is
eligible to become Designated Router, it will select itself as
Designated Router and there will be no Backup Designated Router
for the network.

9.5. Sending Hello packets

Hello packets are sent out each functioning router interface.
They are used to discover and maintain neighbor
relationships.[6] On broadcast and NBMA networks, Hello Packets
are also used to elect the Designated Router and Backup
Designated Router.

The format of an Hello packet is detailed in Section A.3.2. The
Hello Packet contains the router's Router Priority (used in
choosing the Designated Router), and the interval between Hello
Packets sent out the interface (HelloInterval). The Hello
Packet also indicates how often a neighbor must be heard from to
remain active (RouterDeadInterval). Both HelloInterval and
RouterDeadInterval must be the same for all routers attached to
a common network. The Hello packet also contains the IP address
mask of the attached network (Network Mask). On unnumbered
point-to-point networks and on virtual links this field should
be set to 0.0.0.0.

The Hello packet's Options field describes the router's optional
OSPF capabilities. One optional capability is defined in this
specification (see Sections 4.5 and A.2). The E-bit of the
Options field should be set if and only if the attached area is
capable of processing AS-external-LSAs (i.e., it is not a stub
area). If the E-bit is set incorrectly the neighboring routers
will refuse to accept the Hello Packet (see Section 10.5).
Unrecognized bits in the Hello Packet's Options field should be
set to zero.

In order to ensure two-way communication between adjacent
routers, the Hello packet contains the list of all routers on
the network from which Hello Packets have been seen recently.
The Hello packet also contains the router's current choice for
Designated Router and Backup Designated Router. A value of
0.0.0.0 in these fields means that one has not yet been
selected.

On broadcast networks and physical point-to-point networks,
Hello packets are sent every HelloInterval seconds to the IP
multicast address AllSPFRouters. On virtual links, Hello
packets are sent as unicasts (addressed directly to the other
end of the virtual link) every HelloInterval seconds. On Point-
to-MultiPoint networks, separate Hello packets are sent to each
attached neighbor every HelloInterval seconds. Sending of Hello
packets on NBMA networks is covered in the next section.

9.5.1. Sending Hello packets on NBMA networks

Static configuration information may be necessary in order
for the Hello Protocol to function on non-broadcast networks
(see Sections C.5 and C.6). On NBMA networks, every
attached router which is eligible to become Designated
Router becomes aware of all of its neighbors on the network
(either through configuration or by some unspecified
mechanism). Each neighbor is labelled with the neighbor's
Designated Router eligibility.

The interface state must be at least Waiting for any Hello
Packets to be sent out the NBMA interface. Hello Packets
are then sent directly (as unicasts) to some subset of a
router's neighbors. Sometimes an Hello Packet is sent
periodically on a timer; at other times it is sent as a
response to a received Hello Packet. A router's hello-
sending behavior varies depending on whether the router
itself is eligible to become Designated Router.

If the router is eligible to become Designated Router, it
must periodically send Hello Packets to all neighbors that
are also eligible. In addition, if the router is itself the
Designated Router or Backup Designated Router, it must also
send periodic Hello Packets to all other neighbors. This
means that any two eligible routers are always exchanging
Hello Packets, which is necessary for the correct operation
of the Designated Router election algorithm. To minimize
the number of Hello Packets sent, the number of eligible
routers on an NBMA network should be kept small.

If the router is not eligible to become Designated Router,
it must periodically send Hello Packets to both the
Designated Router and the Backup Designated Router (if they
exist). It must also send an Hello Packet in reply to an
Hello Packet received from any eligible neighbor (other than
the current Designated Router and Backup Designated Router).
This is needed to establish an initial bidirectional
relationship with any potential Designated Router.

When sending Hello packets periodically to any neighbor, the
interval between Hello Packets is determined by the

neighbor's state. If the neighbor is in state Down, Hello
Packets are sent every PollInterval seconds. Otherwise,
Hello Packets are sent every HelloInterval seconds.

10. The Neighbor Data Structure

An OSPF router converses with its neighboring routers. Each
separate conversation is described by a "neighbor data structure".
Each conversation is bound to a particular OSPF router interface,
and is identified either by the neighboring router's OSPF Router ID
or by its Neighbor IP address (see below). Thus if the OSPF router
and another router have multiple attached networks in common,
multiple conversations ensue, each described by a unique neighbor
data structure. Each separate conversation is loosely referred to
in the text as being a separate "neighbor".

The neighbor data structure contains all information pertinent to
the forming or formed adjacency between the two neighbors.
(However, remember that not all neighbors become adjacent.) An
adjacency can be viewed as a highly developed conversation between
two routers.

State
The functional level of the neighbor conversation. This is
described in more detail in Section 10.1.

Inactivity Timer
A single shot timer whose firing indicates that no Hello Packet
has been seen from this neighbor recently. The length of the
timer is RouterDeadInterval seconds.

Master/Slave
When the two neighbors are exchanging databases, they form a
master/slave relationship. The master sends the first Database
Description Packet, and is the only part that is allowed to
retransmit. The slave can only respond to the master's Database
Description Packets. The master/slave relationship is
negotiated in state ExStart.

DD Sequence Number
The DD Sequence number of the Database Description packet that
is currently being sent to the neighbor.

Last received Database Description packet
The initialize(I), more (M) and master(MS) bits, Options field,
and DD sequence number contained in the last Database
Description packet received from the neighbor. Used to determine
whether the next Database Description packet received from the
neighbor is a duplicate.

Neighbor ID
The OSPF Router ID of the neighboring router. The Neighbor ID
is learned when Hello packets are received from the neighbor, or
is configured if this is a virtual adjacency (see Section C.4).

Neighbor Priority
The Router Priority of the neighboring router. Contained in the
neighbor's Hello packets, this item is used when selecting the
Designated Router for the attached network.

Neighbor IP address
The IP address of the neighboring router's interface to the
attached network. Used as the Destination IP address when
protocol packets are sent as unicasts along this adjacency.
Also used in router-LSAs as the Link ID for the attached network
if the neighboring router is selected to be Designated Router
(see Section 12.4.1). The Neighbor IP address is learned when
Hello packets are received from the neighbor. For virtual
links, the Neighbor IP address is learned during the routing
table build process (see Section 15).

Neighbor Options
The optional OSPF capabilities supported by the neighbor.
Learned during the Database Exchange process (see Section 10.6).
The neighbor's optional OSPF capabilities are also listed in its
Hello packets. This enables received Hello Packets to be
rejected (i.e., neighbor relationships will not even start to
form) if there is a mismatch in certain crucial OSPF
capabilities (see Section 10.5). The optional OSPF capabilities
are documented in Section 4.5.

Neighbor's Designated Router
The neighbor's idea of the Designated Router. If this is the
neighbor itself, this is important in the local calculation of
the Designated Router. Defined only on broadcast and NBMA
networks.

Neighbor's Backup Designated Router
The neighbor's idea of the Backup Designated Router. If this is
the neighbor itself, this is important in the local calculation
of the Backup Designated Router. Defined only on broadcast and
NBMA networks.

The next set of variables are lists of LSAs. These lists describe
subsets of the area link-state database. This memo defines five
distinct types of LSAs, all of which may be present in an area
link-state database: router-LSAs, network-LSAs, and Type 3 and 4
summary-LSAs (all stored in the area data structure), and AS-
external-LSAs (stored in the global data structure).

Link state retransmission list
The list of LSAs that have been flooded but not acknowledged on
this adjacency. These will be retransmitted at intervals until
they are acknowledged, or until the adjacency is destroyed.

Database summary list
The complete list of LSAs that make up the area link-state
database, at the moment the neighbor goes into Database Exchange
state. This list is sent to the neighbor in Database
Description packets.

Link state request list
The list of LSAs that need to be received from this neighbor in
order to synchronize the two neighbors' link-state databases.
This list is created as Database Description packets are
received, and is then sent to the neighbor in Link State Request
packets. The list is depleted as appropriate Link State Update
packets are received.

10.1. Neighbor states

The state of a neighbor (really, the state of a conversation
being held with a neighboring router) is documented in the
following sections. The states are listed in order of
progressing functionality. For example, the inoperative state
is listed first, followed by a list of intermediate states
before the final, fully functional state is achieved. The
specification makes use of this ordering by sometimes making
references such as "those neighbors/adjacencies in state greater
than X". Figures 12 and 13 show the graph of neighbor state
changes. The arcs of the graphs are labelled with the event
causing the state change. The neighbor events are documented in
Section 10.2.

The graph in Figure 12 shows the state changes effected by the
Hello Protocol. The Hello Protocol is responsible for neighbor
acquisition and maintenance, and for ensuring two way
communication between neighbors.

The graph in Figure 13 shows the forming of an adjacency. Not
every two neighboring routers become adjacent (see Section
10.4). The adjacency starts to form when the neighbor is in
state ExStart. After the two routers discover their
master/slave status, the state transitions to Exchange. At this
point the neighbor starts to be used in the flooding procedure,
and the two neighboring routers begin synchronizing their
databases. When this synchronization is finished, the neighbor
is in state Full and we say that the two routers are fully
adjacent. At this point the adjacency is listed in LSAs.

For a more detailed description of neighbor state changes,
together with the additional actions involved in each change,
see Section 10.3.

Down
This is the initial state of a neighbor conversation. It
indicates that there has been no recent information received
from the neighbor. On NBMA networks, Hello packets may
still be sent to "Down" neighbors, although at a reduced
frequency (see Section 9.5.1).

+----+
Down
+----+

Start
+-------+
Hello +---->Attempt
Received +-------+

+----+<-+ HelloReceived
Init<---------------+
+----+<--------+

2-Way 1-Way
Received Received

+-------+ +-----+
ExStart<--------+------->2-Way
+-------+ +-----+

Figure 12: Neighbor state changes (Hello Protocol)

In addition to the state transitions pictured,
Event KillNbr always forces Down State,
Event InactivityTimer always forces Down State,
Event LLDown always forces Down State

+-------+
ExStart
+-------+

NegotiationDone
+->+--------+
Exchange
+--+--------+

Exchange
Done
+----+ +-------+
Full<---------+----->Loading
+----+<-+ +-------+
LoadingDone
+------------------+

Figure 13: Neighbor state changes (Database Exchange)

In addition to the state transitions pictured,
Event SeqNumberMismatch forces ExStart state,
Event BadLSReq forces ExStart state,
Event 1-Way forces Init state,
Event KillNbr always forces Down State,
Event InactivityTimer always forces Down State,
Event LLDown always forces Down State,
Event AdjOK? leads to adjacency forming/breaking

Attempt
This state is only valid for neighbors attached to NBMA
networks. It indicates that no recent information has been
received from the neighbor, but that a more concerted effort
should be made to contact the neighbor. This is done by
sending the neighbor Hello packets at intervals of
HelloInterval (see Section 9.5.1).

Init
In this state, an Hello packet has recently been seen from
the neighbor. However, bidirectional communication has not
yet been established with the neighbor (i.e., the router
itself did not appear in the neighbor's Hello packet). All

neighbors in this state (or higher) are listed in the Hello
packets sent from the associated interface.

2-Way
In this state, communication between the two routers is
bidirectional. This has been assured by the operation of
the Hello Protocol. This is the most advanced state short
of beginning adjacency establishment. The (Backup)
Designated Router is selected from the set of neighbors in
state 2-Way or greater.

ExStart
This is the first step in creating an adjacency between the
two neighboring routers. The goal of this step is to decide
which router is the master, and to decide upon the initial
DD sequence number. Neighbor conversations in this state or
greater are called adjacencies.

Exchange
In this state the router is describing its entire link state
database by sending Database Description packets to the
neighbor. Each Database Description Packet has a DD
sequence number, and is explicitly acknowledged. Only one
Database Description Packet is allowed outstanding at any
one time. In this state, Link State Request Packets may
also be sent asking for the neighbor's more recent LSAs.
All adjacencies in Exchange state or greater are used by the
flooding procedure. In fact, these adjacencies are fully
capable of transmitting and receiving all types of OSPF
routing protocol packets.

Loading
In this state, Link State Request packets are sent to the
neighbor asking for the more recent LSAs that have been
discovered (but not yet received) in the Exchange state.

Full
In this state, the neighboring routers are fully adjacent.
These adjacencies will now appear in router-LSAs and
network-LSAs.

10.2. Events causing neighbor state changes

State changes can be effected by a number of events. These
events are shown in the labels of the arcs in Figures 12 and 13.
The label definitions are as follows:

HelloReceived
An Hello packet has been received from the neighbor.

Start
This is an indication that Hello Packets should now be sent
to the neighbor at intervals of HelloInterval seconds. This
event is generated only for neighbors associated with NBMA
networks.

2-WayReceived
Bidirectional communication has been realized between the
two neighboring routers. This is indicated by the router
seeing itself in the neighbor's Hello packet.

NegotiationDone
The Master/Slave relationship has been negotiated, and DD
sequence numbers have been exchanged. This signals the
start of the sending/receiving of Database Description
packets. For more information on the generation of this
event, consult Section 10.8.

ExchangeDone
Both routers have successfully transmitted a full sequence
of Database Description packets. Each router now knows what
parts of its link state database are out of date. For more
information on the generation of this event, consult Section
10.8.

BadLSReq
A Link State Request has been received for an LSA not
contained in the database. This indicates an error in the
Database Exchange process.

Loading Done
Link State Updates have been received for all out-of-date

portions of the database. This is indicated by the Link
state request list becoming empty after the Database
Exchange process has completed.

AdjOK?
A decision must be made as to whether an adjacency should be
established/maintained with the neighbor. This event will
start some adjacencies forming, and destroy others.

The following events cause well developed neighbors to revert to
lesser states. Unlike the above events, these events may occur
when the neighbor conversation is in any of a number of states.

SeqNumberMismatch
A Database Description packet has been received that either
a) has an unexpected DD sequence number, b) unexpectedly has
the Init bit set or c) has an Options field differing from
the last Options field received in a Database Description
packet. Any of these conditions indicate that some error
has occurred during adjacency establishment.

1-Way
An Hello packet has been received from the neighbor, in
which the router is not mentioned. This indicates that
communication with the neighbor is not bidirectional.

KillNbr
This is an indication that all communication with the
neighbor is now impossible, forcing the neighbor to
revert to Down state.

InactivityTimer
The inactivity Timer has fired. This means that no Hello
packets have been seen recently from the neighbor. The
neighbor reverts to Down state.

LLDown
This is an indication from the lower level protocols that
the neighbor is now unreachable. For example, on an X.25
network this could be indicated by an X.25 clear indication

with appropriate cause and diagnostic fields. This event
forces the neighbor into Down state.

10.3. The Neighbor state machine

A detailed description of the neighbor state changes follows.
Each state change is invoked by an event (Section 10.2). This
event may produce different effects, depending on the current
state of the neighbor. For this reason, the state machine below
is organized by current neighbor state and received event. Each
entry in the state machine describes the resulting new neighbor
state and the required set of additional actions.

When a neighbor's state changes, it may be necessary to rerun
the Designated Router election algorithm. This is determined by
whether the interface NeighborChange event is generated (see
Section 9.2). Also, if the Interface is in DR state (the router
is itself Designated Router), changes in neighbor state may
cause a new network-LSA to be originated (see Section 12.4).

When the neighbor state machine needs to invoke the interface
state machine, it should be done as a scheduled task (see
Section 4.4). This simplifies things, by ensuring that neither
state machine will be executed recursively.

State(s): Down

Event: Start

New state: Attempt

Action: Send an Hello Packet to the neighbor (this neighbor
is always associated with an NBMA network) and start
the Inactivity Timer for the neighbor. The timer's
later firing would indicate that communication with
the neighbor was not attained.

State(s): Attempt

Event: HelloReceived

New state: Init

Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has now been heard from.

State(s): Down

Event: HelloReceived

New state: Init

Action: Start the Inactivity Timer for the neighbor. The
timer's later firing would indicate that the
neighbor is dead.

State(s): Init or greater

Event: HelloReceived

New state: No state change.

Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has again been heard from.

State(s): Init

Event: 2-WayReceived

New state: Depends upon action routine.

Action: Determine whether an adjacency should be established
with the neighbor (see Section 10.4). If not, the
new neighbor state is 2-Way.

Otherwise (an adjacency should be established) the
neighbor state transitions to ExStart. Upon
entering this state, the router increments the DD

sequence number in the neighbor data structure. If
this is the first time that an adjacency has been
attempted, the DD sequence number should be assigned
some unique value (like the time of day clock). It
then declares itself master (sets the master/slave
bit to master), and starts sending Database
Description Packets, with the initialize (I), more
(M) and master (MS) bits set. This Database
Description Packet should be otherwise empty. This
Database Description Packet should be retransmitted
at intervals of RxmtInterval until the next state is
entered (see Section 10.8).

State(s): ExStart

Event: NegotiationDone

New state: Exchange

Action: The router must list the contents of its entire area
link state database in the neighbor Database summary
list. The area link state database consists of the
router-LSAs, network-LSAs and summary-LSAs contained
in the area structure, along with the AS-external-
LSAs contained in the global structure. AS-
external-LSAs are omitted from a virtual neighbor's
Database summary list. AS-external-LSAs are omitted
from the Database summary list if the area has been
configured as a stub (see Section 3.6). LSAs whose
age is equal to MaxAge are instead added to the
neighbor's Link state retransmission list. A
summary of the Database summary list will be sent to
the neighbor in Database Description packets. Each
Database Description Packet has a DD sequence
number, and is explicitly acknowledged. Only one
Database Description Packet is allowed outstanding
at any one time. For more detail on the sending and
receiving of Database Description packets, see
Sections 10.8 and 10.6.

State(s): Exchange

Event: ExchangeDone

New state: Depends upon action routine.

Action: If the neighbor Link state request list is empty,
the new neighbor state is Full. No other action is
required. This is an adjacency's final state.

Otherwise, the new neighbor state is Loading. Start
(or continue) sending Link State Request packets to
the neighbor (see Section 10.9). These are requests
for the neighbor's more recent LSAs (which were
discovered but not yet received in the Exchange
state). These LSAs are listed in the Link state
request list associated with the neighbor.

State(s): Loading

Event: Loading Done

New state: Full

Action: No action required. This is an adjacency's final
state.

State(s): 2-Way

Event: AdjOK?

New state: Depends upon action routine.

Action: Determine whether an adjacency should be formed with
the neighboring router (see Section 10.4). If not,
the neighbor state remains at 2-Way. Otherwise,
transition the neighbor state to ExStart and perform
the actions associated with the above state machine
entry for state Init and event 2-WayReceived.

State(s): ExStart or greater

Event: AdjOK?

New state: Depends upon action routine.

Action: Determine whether the neighboring router should
still be adjacent. If yes, there is no state change
and no further action is necessary.

Otherwise, the (possibly partially formed) adjacency
must be destroyed. The neighbor state transitions
to 2-Way. The Link state retransmission list,
Database summary list and Link state request list
are cleared of LSAs.

State(s): Exchange or greater

Event: SeqNumberMismatch

New state: ExStart

Action: The (possibly partially formed) adjacency is torn
down, and then an attempt is made at
reestablishment. The neighbor state first
transitions to ExStart. The Link state
retransmission list, Database summary list and Link
state request list are cleared of LSAs. Then the
router increments the DD sequence number in the
neighbor data structure, declares itself master
(sets the master/slave bit to master), and starts
sending Database Description Packets, with the
initialize (I), more (M) and master (MS) bits set.
This Database Description Packet should be otherwise
empty (see Section 10.8).

State(s): Exchange or greater

Event: BadLSReq

New state: ExStart

Action: The action for event BadLSReq is exactly the same as
for the neighbor event SeqNumberMismatch. The
(possibly partially formed) adjacency is torn down,
and then an attempt is made at reestablishment. For
more information, see the neighbor state machine
entry that is invoked when event SeqNumberMismatch
is generated in state Exchange or greater.

State(s): Any state

Event: KillNbr

New state: Down

Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs. Also, the Inactivity Timer is disabled.

State(s): Any state

Event: LLDown

New state: Down

Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs. Also, the Inactivity Timer is disabled.

State(s): Any state

Event: InactivityTimer

New state: Down

Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs.

State(s): 2-Way or greater

Event: 1-WayReceived

New state: Init

Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of
LSAs.

State(s): 2-Way or greater

Event: 2-WayReceived

New state: No state change.

Action: No action required.

State(s): Init

Event: 1-WayReceived

New state: No state change.

Action: No action required.

10.4. Whether to become adjacent

Adjacencies are established with some subset of the router's
neighbors. Routers connected by point-to-point networks,
Point-to-MultiPoint networks and virtual links always become
adjacent. On broadcast and NBMA networks, all routers become
adjacent to both the Designated Router and the Backup Designated
Router.

The adjacency-forming decision occurs in two places in the
neighbor state machine. First, when bidirectional communication
is initially established with the neighbor, and secondly, when
the identity of the attached network's (Backup) Designated

Router changes. If the decision is made to not attempt an
adjacency, the state of the neighbor communication stops at 2-
Way.

An adjacency should be established with a bidirectional neighbor
when at least one of the following conditions holds:

o The underlying network type is point-to-point

o The underlying network type is Point-to-MultiPoint

o The underlying network type is virtual link

o The router itself is the Designated Router

o The router itself is the Backup Designated Router

o The neighboring router is the Designated Router

o The neighboring router is the Backup Designated Router

10.5. Receiving Hello Packets

This section explains the detailed processing of a received
Hello Packet. (See Section A.3.2 for the format of Hello
packets.) The generic input processing of OSPF packets will
have checked the validity of the IP header and the OSPF packet
header. Next, the values of the Network Mask, HelloInterval,
and RouterDeadInterval fields in the received Hello packet must
be checked against the values configured for the receiving
interface. Any mismatch causes processing to stop and the
packet to be dropped. In other words, the above fields are
really describing the attached network's configuration. However,
there is one exception to the above rule: on point-to-point
networks and on virtual links, the Network Mask in the received
Hello Packet should be ignored.

The receiving interface attaches to a single OSPF area (this
could be the backbone). The setting of the E-bit found in the
Hello Packet's Options field must match this area's

ExternalRoutingCapability. If AS-external-LSAs are not flooded
into/throughout the area (i.e, the area is a "stub") the E-bit
must be clear in received Hello Packets, otherwise the E-bit
must be set. A mismatch causes processing to stop and the
packet to be dropped. The setting of the rest of the bits in
the Hello Packet's Options field should be ignored.

At this point, an attempt is made to match the source of the
Hello Packet to one of the receiving interface's neighbors. If
the receiving interface connects to a broadcast, Point-to-
MultiPoint or NBMA network the source is identified by the IP
source address found in the Hello's IP header. If the receiving
interface connects to a point-to-point link or a virtual link,
the source is identified by the Router ID found in the Hello's
OSPF packet header. The interface's current list of neighbors
is contained in the interface's data structure. If a matching
neighbor structure cannot be found, (i.e., this is the first
time the neighbor has been detected), one is created. The
initial state of a newly created neighbor is set to Down.

When receiving an Hello Packet from a neighbor on a broadcast,
Point-to-MultiPoint or NBMA network, set the neighbor
structure's Neighbor ID equal to the Router ID found in the
packet's OSPF header. For these network types, the neighbor
structure's Router Priority field, Neighbor's Designated Router
field, and Neighbor's Backup Designated Router field are also
set equal to the corresponding fields found in the received
Hello Packet; changes in these fields should be noted for
possible use in the steps below. When receiving an Hello on a
point-to-point network (but not on a virtual link) set the
neighbor structure's Neighbor IP address to the packet's IP
source address.

Now the rest of the Hello Packet is examined, generating events
to be given to the neighbor and interface state machines. These
state machines are specified either to be executed or scheduled
(see Section 4.4). For example, by specifying below that the
neighbor state machine be executed in line, several neighbor
state transitions may be effected by a single received Hello:

o Each Hello Packet causes the neighbor state machine to be
executed with the event HelloReceived.

o Then the list of neighbors contained in the Hello Packet is
examined. If the router itself appears in this list, the
neighbor state machine should be executed with the event 2-
WayReceived. Otherwise, the neighbor state machine should
be executed with the event 1-WayReceived, and the processing
of the packet stops.

o Next, if a change in the neighbor's Router Priority field
was noted, the receiving interface's state machine is
scheduled with the event NeighborChange.

o If the neighbor is both declaring itself to be Designated
Router (Hello Packet's Designated Router field = Neighbor IP
address) and the Backup Designated Router field in the
packet is equal to 0.0.0.0 and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Designated Router and it
had not previously, or the neighbor is not declaring itself
Designated Router where it had previously, the receiving
interface's state machine is scheduled with the event
NeighborChange.

o If the neighbor is declaring itself to be Backup Designated
Router (Hello Packet's Backup Designated Router field =
Neighbor IP address) and the receiving interface is in state
Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Backup Designated Router
and it had not previously, or the neighbor is not declaring
itself Backup Designated Router where it had previously, the
receiving interface's state machine is scheduled with the
event NeighborChange.

On NBMA networks, receipt of an Hello Packet may also cause an
Hello Packet to be sent back to the neighbor in response. See
Section 9.5.1 for more details.

10.6. Receiving Database Description Packets

This section explains the detailed processing of a received
Database Description Packet. The incoming Database Description
Packet has already been associated with a neighbor and receiving
interface by the generic input packet processing (Section 8.2).
Whether the Database Description packet should be accepted, and
if so, how it should be further processed depends upon the
neighbor state.

If a Database Description packet is accepted, the following
packet fields should be saved in the corresponding neighbor data
structure under "last received Database Description packet":
the packet's initialize(I), more (M) and master(MS) bits,
Options field, and DD sequence number. If these fields are set
identically in two consecutive Database Description packets
received from the neighbor, the second Database Description
packet is considered to be a "duplicate" in the processing
described below.

If the Interface MTU field in the Database Description packet
indicates an IP datagram size that is larger than the router can
accept on the receiving interface without fragmentation, the
Database Description packet is rejected. Otherwise, if the
neighbor state is:

Down
The packet should be rejected.

Attempt
The packet should be rejected.

Init
The neighbor state machine should be executed with the event
2-WayReceived. This causes an immediate state change to
either state 2-Way or state ExStart. If the new state is
ExStart, the processing of the current packet should then
continue in this new state by falling through to case
ExStart below.

2-Way
The packet should be ignored. Database Description Packets
are used only for the purpose of bringing up adjacencies.[7]

ExStart
If the received packet matches one of the following cases,
then the neighbor state machine should be executed with the
event NegotiationDone (causing the state to transition to
Exchange), the packet's Options field should be recorded in
the neighbor structure's Neighbor Options field and the
packet should be accepted as next in sequence and processed
further (see below). Otherwise, the packet should be
ignored.

o The initialize(I), more (M) and master(MS) bits are set,
the contents of the packet are empty, and the neighbor's
Router ID is larger than the router's own. In this case
the router is now Slave. Set the master/slave bit to
slave, and set the neighbor data structure's DD sequence
number to that specified by the master.

o The initialize(I) and master(MS) bits are off, the
packet's DD sequence number equals the neighbor data
structure's DD sequence number (indicating
acknowledgment) and the neighbor's Router ID is smaller
than the router's own. In this case the router is
Master.

Exchange
Duplicate Database Description packets are discarded by the
master, and cause the slave to retransmit the last Database
Description packet that it had sent. Otherwise (the packet
is not a duplicate):

o If the state of the MS-bit is inconsistent with the
master/slave state of the connection, generate the
neighbor event SeqNumberMismatch and stop processing the
packet.

o If the initialize(I) bit is set, generate the neighbor
event SeqNumberMismatch and stop processing the packet.

o If the packet's Options field indicates a different set
of optional OSPF capabilities than were previously
received from the neighbor (recorded in the Neighbor
Options field of the neighbor structure), generate the
neighbor event SeqNumberMismatch and stop processing the
packet.

o Database Description packets must be processed in
sequence, as indicated by the packets' DD sequence
numbers. If the router is master, the next packet
received should have DD sequence number equal to the DD
sequence number in the neighbor data structure. If the
router is slave, the next packet received should have DD
sequence number equal to one more than the DD sequence
number stored in the neighbor data structure. In either
case, if the packet is the next in sequence it should be
accepted and its contents processed as specified below.

o Else, generate the neighbor event SeqNumberMismatch and
stop processing the packet.

Loading or Full
In this state, the router has sent and received an entire
sequence of Database Description Packets. The only packets
received should be duplicates (see above). In particular,
the packet's Options field should match the set of optional
OSPF capabilities previously indicated by the neighbor
(stored in the neighbor structure's Neighbor Options field).
Any other packets received, including the reception of a
packet with the Initialize(I) bit set, should generate the
neighbor event SeqNumberMismatch.[8] Duplicates should be
discarded by the master. The slave must respond to
duplicates by repeating the last Database Description packet
that it had sent.

When the router accepts a received Database Description Packet
as the next in sequence the packet contents are processed as
follows. For each LSA listed, the LSA's LS type is checked for
validity. If the LS type is unknown (e.g., not one of the LS
types 1-5 defined by this specification), or if this is an AS-
external-LSA (LS type = 5) and the neighbor is associated with a

stub area, generate the neighbor event SeqNumberMismatch and
stop processing the packet. Otherwise, the router looks up the
LSA in its database to see whether it also has an instance of
the LSA. If it does not, or if the database copy is less recent
(see Section 13.1), the LSA is put on the Link state request
list so that it can be requested (immediately or at some later
time) in Link State Request Packets.

When the router accepts a received Database Description Packet
as the next in sequence, it also performs the following actions,
depending on whether it is master or slave:

Master
Increments the DD sequence number in the neighbor data
structure. If the router has already sent its entire
sequence of Database Description Packets, and the just
accepted packet has the more bit (M) set to 0, the neighbor
event ExchangeDone is generated. Otherwise, it should send
a new Database Description to the slave.

Slave
Sets the DD sequence number in the neighbor data structure
to the DD sequence number appearing in the received packet.
The slave must send a Database Description Packet in reply.
If the received packet has the more bit (M) set to 0, and
the packet to be sent by the slave will also have the M-bit
set to 0, the neighbor event ExchangeDone is generated.
Note that the slave always generates this event before the
master.

10.7. Receiving Link State Request Packets

This section explains the detailed processing of received Link
State Request packets. Received Link State Request Packets
specify a list of LSAs that the neighbor wishes to receive.
Link State Request Packets should be accepted when the neighbor
is in states Exchange, Loading, or Full. In all other states
Link State Request Packets should be ignored.

Each LSA specified in the Link State Request packet should be
located in the router's database, and copied into Link State
Update packets for transmission to the neighbor. These LSAs
should NOT be placed on the Link state retransmission list for
the neighbor. If an LSA cannot be found in the database,
something has gone wrong with the Database Exchange process, and
neighbor event BadLSReq should be generated.

10.8. Sending Database Description Packets

This section describes how Database Description Packets are sent
to a neighbor. The Database Description packet's Interface MTU
field is set to the size of the largest IP datagram that can be
sent out the sending interface, without fragmentation. Common
MTUs in use in the Internet can be found in Table 7-1 of
[Ref22]. Interface MTU should be set to 0 in Database
Description packets sent over virtual links.

The router's optional OSPF capabilities (see Section 4.5) are
transmitted to the neighbor in the Options field of the Database
Description packet. The router should maintain the same set of
optional capabilities throughout the Database Exchange and
flooding procedures. If for some reason the router's optional
capabilities change, the Database Exchange procedure should be
restarted by reverting to neighbor state ExStart. One optional
capability is defined in this specification (see Sections 4.5
and A.2). The E-bit should be set if and only if the attached
network belongs to a non-stub area. Unrecognized bits in the
Options field should be set to zero.

The sending of Database Description packets depends on the
neighbor's state. In state ExStart the router sends empty
Database Description packets, with the initialize (I), more (M)
and master (MS) bits set. These packets are retransmitted every
RxmtInterval seconds.

In state Exchange the Database Description Packets actually
contain summaries of the link state information contained in the
router's database. Each LSA in the area's link-state database
(at the time the neighbor transitions into Exchange state) is
listed in the neighbor Database summary list. Each new Database

Description Packet copies its DD sequence number from the
neighbor data structure and then describes the current top of
the Database summary list. Items are removed from the Database
summary list when the previous packet is acknowledged.

In state Exchange, the determination of when to send a Database
Description packet depends on whether the router is master or
slave:

Master
Database Description packets are sent when either a) the
slave acknowledges the previous Database Description packet
by echoing the DD sequence number or b) RxmtInterval seconds
elapse without an acknowledgment, in which case the previous
Database Description packet is retransmitted.

Slave
Database Description packets are sent only in response to
Database Description packets received from the master. If
the Database Description packet received from the master is
new, a new Database Description packet is sent, otherwise
the previous Database Description packet is resent.

In states Loading and Full the slave must resend its last
Database Description packet in response to duplicate Database
Description packets received from the master. For this reason
the slave must wait RouterDeadInterval seconds before freeing
the last Database Description packet. Reception of a Database
Description packet from the master after this interval will
generate a SeqNumberMismatch neighbor event.

10.9. Sending Link State Request Packets

In neighbor states Exchange or Loading, the Link state request
list contains a list of those LSAs that need to be obtained from
the neighbor. To request these LSAs, a router sends the
neighbor the beginning of the Link state request list, packaged
in a Link State Request packet.

When the neighbor responds to these requests with the proper
Link State Update packet(s), the Link state request list is
truncated and a new Link State Request packet is sent. This
process continues until the Link state request list becomes
empty. LSAs on the Link state request list that have been
requested, but not yet received, are packaged into Link State
Request packets for retransmission at intervals of RxmtInterval.
There should be at most one Link State Request packet
outstanding at any one time.

When the Link state request list becomes empty, and the neighbor
state is Loading (i.e., a complete sequence of Database
Description packets has been sent to and received from the
neighbor), the Loading Done neighbor event is generated.

10.10. An Example

Figure 14 shows an example of an adjacency forming. Routers RT1
and RT2 are both connected to a broadcast network. It is
assumed that RT2 is the Designated Router for the network, and
that RT2 has a higher Router ID than Router RT1.

The neighbor state changes realized by each router are listed on
the sides of the figure.

At the beginning of Figure 14, Router RT1's interface to the
network becomes operational. It begins sending Hello Packets,
although it doesn't know the identity of the Designated Router
or of any other neighboring routers. Router RT2 hears this
hello (moving the neighbor to Init state), and in its next Hello
Packet indicates that it is itself the Designated Router and
that it has heard Hello Packets from RT1. This in turn causes
RT1 to go to state ExStart, as it starts to bring up the
adjacency.

RT1 begins by asserting itself as the master. When it sees that
RT2 is indeed the master (because of RT2's higher Router ID),
RT1 transitions to slave state and adopts its neighbor's DD
sequence number. Database Description packets are then
exchanged, with polls coming from the master (RT2) and responses
from the slave (RT1). This sequence of Database Description

+---+ +---+
RT1 RT2
+---+ +---+

Down Down
Hello(DR=0,seen=0)
------------------------------>
Hello (DR=RT2,seen=RT1,...) Init
<------------------------------
ExStart D-D (Seq=x,I,M,Master)
------------------------------>
D-D (Seq=y,I,M,Master) ExStart
<------------------------------
Exchange D-D (Seq=y,M,Slave)
------------------------------>
D-D (Seq=y+1,M,Master) Exchange
<------------------------------
D-D (Seq=y+1,M,Slave)
------------------------------>
...
...
...
D-D (Seq=y+n, Master)
<------------------------------
D-D (Seq=y+n, Slave)
Loading ------------------------------>
LS Request Full
------------------------------>
LS Update
<------------------------------
LS Request
------------------------------>
LS Update
<------------------------------
Full

Figure 14: An adjacency bring-up example

Packets ends when both the poll and associated response has the
M-bit off.

In this example, it is assumed that RT2 has a completely up to
date database. In that case, RT2 goes immediately into Full
state. RT1 will go into Full state after updating the necessary
parts of its database. This is done by sending Link State
Request Packets, and receiving Link State Update Packets in
response. Note that, while RT1 has waited until a complete set
of Database Description Packets has been received (from RT2)
before sending any Link State Request Packets, this need not be
the case. RT1 could have interleaved the sending of Link State
Request Packets with the reception of Database Description
Packets.

11. The Routing Table Structure

The routing table data structure contains all the information
necessary to forward an IP data packet toward its destination. Each
routing table entry describes the collection of best paths to a
particular destination. When forwarding an IP data packet, the
routing table entry providing the best match for the packet's IP
destination is located. The matching routing table entry then
provides the next hop towards the packet's destination. OSPF also
provides for the existence of a default route (Destination ID =
DefaultDestination, Address Mask = 0x00000000). When the default
route exists, it matches all IP destinations (although any other
matching entry is a better match). Finding the routing table entry
that best matches an IP destination is further described in Section
11.1.

There is a single routing table in each router. Two sample routing
tables are described in Sections 11.2 and 11.3. The building of the
routing table is discussed in Section 16.

The rest of this section defines the fields found in a routing table
entry. The first set of fields describes the routing table entry's
destination.

Destination Type
Destination type is either "network" or "router". Only network
entries are actually used when forwarding IP data traffic.
Router routing table entries are used solely as intermediate
steps in the routing table build process.

A network is a range of IP addresses, to which IP data traffic
may be forwarded. This includes IP networks (class A, B, or C),
IP subnets, IP supernets and single IP hosts. The default route
also falls into this category.

Router entries are kept for area border routers and AS boundary
routers. Routing table entries for area border routers are used
when calculating the inter-area routes (see Section 16.2), and
when maintaining configured virtual links (see Section 15).
Routing table entries for AS boundary routers are used when
calculating the AS external routes (see Section 16.4).

Destination ID
The destination's identifier or name. This depends on the
Destination Type. For networks, the identifier is their
associated IP address. For routers, the identifier is the OSPF
Router ID.[9]

Address Mask
Only defined for networks. The network's IP address together
with its address mask defines a range of IP addresses. For IP
subnets, the address mask is referred to as the subnet mask.
For host routes, the mask is "all ones" (0xffffffff).

Optional Capabilities
When the destination is a router this field indicates the
optional OSPF capabilities supported by the destination router.
The only optional capability defined by this specification is
the ability to process AS-external-LSAs. For a further
discussion of OSPF's optional capabilities, see Section 4.5.

The set of paths to use for a destination may vary based on the OSPF
area to which the paths belong. This means that there may be
multiple routing table entries for the same destination, depending
on the values of the next field.

Area
This field indicates the area whose link state information has
led to the routing table entry's collection of paths. This is
called the entry's associated area. For sets of AS external
paths, this field is not defined. For destinations of type
"router", there may be separate sets of paths (and therefore
separate routing table entries) associated with each of several
areas. For example, this will happen when two area border
routers share multiple areas in common. For destinations of
type "network", only the set of paths associated with the best
area (the one providing the preferred route) is kept.

The rest of the routing table entry describes the set of paths to
the destination. The following fields pertain to the set of paths
as a whole. In other words, each one of the paths contained in a
routing table entry is of the same path-type and cost (see below).

Path-type
There are four possible types of paths used to route traffic to
the destination, listed here in decreasing order of preference:
intra-area, inter-area, type 1 external or type 2 external.
Intra-area paths indicate destinations belonging to one of the
router's attached areas. Inter-area paths are paths to
destinations in other OSPF areas. These are discovered through
the examination of received summary-LSAs. AS external paths are
paths to destinations external to the AS. These are detected
through the examination of received AS-external-LSAs.

Cost
The link state cost of the path to the destination. For all
paths except type 2 external paths this describes the entire
path's cost. For Type 2 external paths, this field describes
the cost of the portion of the path internal to the AS. This

cost is calculated as the sum of the costs of the path's
constituent links.

Type 2 cost
Only valid for type 2 external paths. For these paths, this
field indicates the cost of the path's external portion. This
cost has been advertised by an AS boundary router, and is the
most significant part of the total path cost. For example, a
type 2 external path with type 2 cost of 5 is always preferred
over a path with type 2 cost of 10, regardless of the cost of
the two paths' internal components.

Link State Origin
Valid only for intra-area paths, this field indicates the LSA
(router-LSA or network-LSA) that directly references the
destination. For example, if the destination is a transit
network, this is the transit network's network-LSA. If the
destination is a stub network, this is the router-LSA for the
attached router. The LSA is discovered during the shortest-path
tree calculation (see Section 16.1). Multiple LSAs may
reference the destination, however a tie-breaking scheme always
reduces the choice to a single LSA. The Link State Origin field
is not used by the OSPF protocol, but it is used by the routing
table calculation in OSPF's Multicast routing extensions
(MOSPF).

When multiple paths of equal path-type and cost exist to a
destination (called elsewhere "equal-cost" paths), they are stored
in a single routing table entry. Each one of the "equal-cost" paths
is distinguished by the following fields:

Next hop
The outgoing router interface to use when forwarding traffic to
the destination. On broadcast, Point-to-MultiPoint and NBMA
networks, the next hop also includes the IP address of the next
router (if any) in the path towards the destination.

Advertising router
Valid only for inter-area and AS external paths. This field
indicates the Router ID of the router advertising the summary-
LSA or AS-external-LSA that led to this path.

11.1. Routing table lookup

When an IP data packet is received, an OSPF router finds the
routing table entry that best matches the packet's destination.
This routing table entry then provides the outgoing interface
and next hop router to use in forwarding the packet. This
section describes the process of finding the best matching
routing table entry.

Before the lookup begins, "discard" routing table entries should
be inserted into the routing table for each of the router's
active area address ranges (see Section 3.5). (An area range is
considered "active" if the range contains one or more networks
reachable by intra-area paths.) The destination of a "discard"
entry is the set of addresses described by its associated active
area address range, and the path type of each "discard" entry is
set to "inter-area".[10]

Several routing table entries may match the destination address.
In this case, the "best match" is the routing table entry that
provides the most specific (longest) match. Another way of
saying this is to choose the entry that specifies the narrowest
range of IP addresses.[11] For example, the entry for the
address/mask pair of (128.185.1.0, 0xffffff00) is more specific
than an entry for the pair (128.185.0.0, 0xffff0000). The
default route is the least specific match, since it matches all
destinations. (Note that for any single routing table entry,
multiple paths may be possible. In these cases, the calculations
in Sections 16.1, 16.2, and 16.4 always yield the paths having
the most preferential path-type, as described in Section 11).

If there is no matching routing table entry, or the best match
routing table entry is one of the above "discard" routing table
entries, then the packet's IP destination is considered
unreachable. Instead of being forwarded, the packet should then
be discarded and an ICMP destination unreachable message should
be returned to the packet's source.

11.2. Sample routing table, without areas

Consider the Autonomous System pictured in Figure 2. No OSPF
areas have been configured. A single metric is shown per

outbound interface. The calculation of Router RT6's routing
table proceeds as described in Section 2.2. The resulting
routing table is shown in Table 12. Destination types are
abbreviated: Network as "N", Router as "R".

There are no instances of multiple equal-cost shortest paths in
this example. Also, since there are no areas, there are no
inter-area paths.

Routers RT5 and RT7 are AS boundary routers. Intra-area routes
have been calculated to Routers RT5 and RT7. This allows
external routes to be calculated to the destinations advertised
by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15). It is
assumed all AS-external-LSAs originated by RT5 and RT7 are
advertising type 1 external metrics. This results in type 1
external paths being calculated to destinations N12-N15.

11.3. Sample routing table, with areas

Consider the previous example, this time split into OSPF areas.
An OSPF area configuration is pictured in Figure 6. Router
RT4's routing table will be described for this area
configuration. Router RT4 has a connection to Area 1 and a
backbone connection. This causes Router RT4 to view the AS as
the concatenation of the two graphs shown in Figures 7 and 8.
The resulting routing table is displayed in Table 13.

Again, Routers RT5 and RT7 are AS boundary routers. Routers
RT3, RT4, RT7, RT10 and RT11 are area border routers. Note that
there are two routing entries for the area border router RT3,
since it has two areas in common with RT4 (Area 1 and the
backbone).

Backbone paths have been calculated to all area border routers.
These are used when determining the inter-area routes. Note
that all of the inter-area routes are associated with the
backbone; this is always the case when the calculating router is
itself an area border router. Routing information is condensed
at area boundaries. In this example, we assume that Area 3 has
been defined so that networks N9-N11 and the host route to H1

Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
____________________________________________________________
N N1 0 intra-area 10 RT3 *
N N2 0 intra-area 10 RT3 *
N N3 0 intra-area 7 RT3 *
N N4 0 intra-area 8 RT3 *
N Ib 0 intra-area 7 * *
N Ia 0 intra-area 12 RT10 *
N N6 0 intra-area 8 RT10 *
N N7 0 intra-area 12 RT10 *
N N8 0 intra-area 10 RT10 *
N N9 0 intra-area 11 RT10 *
N N10 0 intra-area 13 RT10 *
N N11 0 intra-area 14 RT10 *
N H1 0 intra-area 21 RT10 *
R RT5 0 intra-area 6 RT5 *
R RT7 0 intra-area 8 RT10 *
____________________________________________________________
N N12 * type 1 ext. 10 RT10 RT7
N N13 * type 1 ext. 14 RT5 RT5
N N14 * type 1 ext. 14 RT5 RT5
N N15 * type 1 ext. 17 RT10 RT7

Table 12: The routing table for Router RT6
(no configured areas).

are all condensed to a single route when advertised into the
backbone (by Router RT11). Note that the cost of this route is
the maximum of the set of costs to its individual components.

There is a virtual link configured between Routers RT10 and
RT11. Without this configured virtual link, RT11 would be
unable to advertise a route for networks N9-N11 and Host H1 into
the backbone, and there would not be an entry for these networks
in Router RT4's routing table.

In this example there are two equal-cost paths to Network N12.
However, they both use the same next hop (Router RT5).

Router RT4's routing table would improve (i.e., some of the
paths in the routing table would become shorter) if an
additional virtual link were configured between Router RT4 and
Router RT3. The new virtual link would itself be associated
with the first entry for area border router RT3 in Table 13 (an
intra-area path through Area 1). This would yield a cost of 1
for the virtual link. The routing table entries changes that
would be caused by the addition of this virtual link are shown

Type Dest Area Path Type Cost Next Adv.
Hops(s) Router(s)
__________________________________________________________________
N N1 1 intra-area 4 RT1 *
N N2 1 intra-area 4 RT2 *
N N3 1 intra-area 1 * *
N N4 1 intra-area 3 RT3 *
R RT3 1 intra-area 1 * *
__________________________________________________________________
N Ib 0 intra-area 22 RT5 *
N Ia 0 intra-area 27 RT5 *
R RT3 0 intra-area 21 RT5 *
R RT5 0 intra-area 8 * *
R RT7 0 intra-area 14 RT5 *
R RT10 0 intra-area 22 RT5 *
R RT11 0 intra-area 25 RT5 *
__________________________________________________________________
N N6 0 inter-area 15 RT5 RT7
N N7 0 inter-area 19 RT5 RT7
N N8 0 inter-area 18 RT5 RT7
N N9-N11,H1 0 inter-area 36 RT5 RT11
__________________________________________________________________
N N12 * type 1 ext. 16 RT5 RT5,RT7
N N13 * type 1 ext. 16 RT5 RT5
N N14 * type 1 ext. 16 RT5 RT5
N N15 * type 1 ext. 23 RT5 RT7

Table 13: Router RT4's routing table
in the presence of areas.

in Table 14.

12. Link State Advertisements (LSAs)

Each router in the Autonomous System originates one or more link
state advertisements (LSAs). This memo defines five distinct types
of LSAs, which are described in Section 4.3. The collection of LSAs
forms the link-state database. Each separate type of LSA has a
separate function. Router-LSAs and network-LSAs describe how an
area's routers and networks are interconnected. Summary-LSAs
provide a way of condensing an area's routing information. AS-
external-LSAs provide a way of transparently advertising
externally-derived routing information throughout the Autonomous
System.

Each LSA begins with a standard 20-byte header. This LSA header is
discussed below.

Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
R RT3 0 intra-area 1 * *
R RT10 0 intra-area 16 RT3 *
R RT11 0 intra-area 19 RT3 *
________________________________________________________________
N N9-N11,H1 0 inter-area 30 RT3 RT11

Table 14: Changes resulting from an
additional virtual link.

12.1. The LSA Header

The LSA header contains the LS type, Link State ID and
Advertising Router fields. The combination of these three
fields uniquely identifies the LSA.

There may be several instances of an LSA present in the
Autonomous System, all at the same time. It must then be
determined which instance is more recent. This determination is
made by examining the LS sequence, LS checksum and LS age
fields. These fields are also contained in the 20-byte LSA
header.

Several of the OSPF packet types list LSAs. When the instance
is not important, an LSA is referred to by its LS type, Link
State ID and Advertising Router (see Link State Request
Packets). Otherwise, the LS sequence number, LS age and LS
checksum fields must also be referenced.

A detailed explanation of the fields contained in the LSA header
follows.

12.1.1. LS age

This field is the age of the LSA in seconds. It should be
processed as an unsigned 16-bit integer. It is set to 0
when the LSA is originated. It must be incremented by
InfTransDelay on every hop of the flooding procedure. LSAs
are also aged as they are held in each router's database.

The age of an LSA is never incremented past MaxAge. LSAs
having age MaxAge are not used in the routing table
calculation. When an LSA's age first reaches MaxAge, it is
reflooded. An LSA of age MaxAge is finally flushed from the
database when it is no longer needed to ensure database
synchronization. For more information on the aging of LSAs,
consult Section 14.

The LS age field is examined when a router receives two
instances of an LSA, both having identical LS sequence
numbers and LS checksums. An instance of age MaxAge is then

always accepted as most recent; this allows old LSAs to be
flushed quickly from the routing domain. Otherwise, if the
ages differ by more than MaxAgeDiff, the instance having the
smaller age is accepted as most recent.[12] See Section 13.1
for more details.

12.1.2. Options

The Options field in the LSA header indicates which optional
capabilities are associated with the LSA. OSPF's optional
capabilities are described in Section 4.5. One optional
capability is defined by this specification, represented by
the E-bit found in the Options field. The unrecognized bits
in the Options field should be set to zero.

The E-bit represents OSPF's ExternalRoutingCapability. This
bit should be set in all LSAs associated with the backbone,
and all LSAs associated with non-stub areas (see Section
3.6). It should also be set in all AS-external-LSAs. It
should be reset in all router-LSAs, network-LSAs and
summary-LSAs associated with a stub area. For all LSAs, the
setting of the E-bit is for informational purposes only; it
does not affect the routing table calculation.

12.1.3. LS type

The LS type field dictates the format and function of the
LSA. LSAs of different types have different names (e.g.,
router-LSAs or network-LSAs). All LSA types defined by this
memo, except the AS-external-LSAs (LS type = 5), are flooded
throughout a single area only. AS-external-LSAs are flooded
throughout the entire Autonomous System, excepting stub
areas (see Section 3.6). Each separate LSA type is briefly
described below in Table 15.

12.1.4. Link State ID

This field identifies the piece of the routing domain that
is being described by the LSA. Depending on the LSA's LS
type, the Link State ID takes on the values listed in Table

LS Type LSA description
________________________________________________
1 These are the router-LSAs.
They describe the collected
states of the router's
interfaces. For more information,
consult Section 12.4.1.
________________________________________________
2 These are the network-LSAs.
They describe the set of routers
attached to the network. For
more information, consult
Section 12.4.2.
________________________________________________
3 or 4 These are the summary-LSAs.
They describe inter-area routes,
and enable the condensation of
routing information at area
borders. Originated by area border
routers, the Type 3 summary-LSAs
describe routes to networks while the
Type 4 summary-LSAs describe routes to
AS boundary routers.
________________________________________________
5 These are the AS-external-LSAs.
Originated by AS boundary routers,
they describe routes
to destinations external to the
Autonomous System. A default route for
the Autonomous System can also be
described by an AS-external-LSA.

Table 15: OSPF link state advertisements (LSAs).

16.

Actually, for Type 3 summary-LSAs (LS type = 3) and AS-
external-LSAs (LS type = 5), the Link State ID may

LS Type Link State ID
_______________________________________________
1 The originating router's Router ID.
2 The IP interface address of the
network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS
boundary router.
5 The destination network's IP address.

Table 16: The LSA's Link State ID.

additionally have one or more of the destination network's
"host" bits set. For example, when originating an AS-
external-LSA for the network 10.0.0.0 with mask of
255.0.0.0, the Link State ID can be set to anything in the
range 10.0.0.0 through 10.255.255.255 inclusive (although
10.0.0.0 should be used whenever possible). The freedom to
set certain host bits allows a router to originate separate
LSAs for two networks having the same address but different
masks. See Appendix E for details.

When the LSA is describing a network (LS type = 2, 3 or 5),
the network's IP address is easily derived by masking the
Link State ID with the network/subnet mask contained in the
body of the LSA. When the LSA is describing a router (LS
type = 1 or 4), the Link State ID is always the described
router's OSPF Router ID.

When an AS-external-LSA (LS Type = 5) is describing a
default route, its Link State ID is set to
DefaultDestination (0.0.0.0).

12.1.5. Advertising Router

This field specifies the OSPF Router ID of the LSA's
originator. For router-LSAs, this field is identical to the
Link State ID field. Network-LSAs are originated by the

network's Designated Router. Summary-LSAs originated by
area border routers. AS-external-LSAs are originated by AS
boundary routers.

12.1.6. LS sequence number

The sequence number field is a signed 32-bit integer. It is
used to detect old and duplicate LSAs. The space of
sequence numbers is linearly ordered. The larger the
sequence number (when compared as signed 32-bit integers)
the more recent the LSA. To describe to sequence number
space more precisely, let N refer in the discussion below to
the constant 2**31.

The sequence number -N (0x80000000) is reserved (and
unused). This leaves -N + 1 (0x80000001) as the smallest
(and therefore oldest) sequence number; this sequence number
is referred to as the constant InitialSequenceNumber. A
router uses InitialSequenceNumber the first time it
originates any LSA. Afterwards, the LSA's sequence number
is incremented each time the router originates a new
instance of the LSA. When an attempt is made to increment
the sequence number past the maximum value of N - 1
(0x7fffffff; also referred to as MaxSequenceNumber), the
current instance of the LSA must first be flushed from the
routing domain. This is done by prematurely aging the LSA
(see Section 14.1) and reflooding it. As soon as this flood
has been acknowledged by all adjacent neighbors, a new
instance can be originated with sequence number of
InitialSequenceNumber.

The router may be forced to promote the sequence number of
one of its LSAs when a more recent instance of the LSA is
unexpectedly received during the flooding process. This
should be a rare event. This may indicate that an out-of-
date LSA, originated by the router itself before its last
restart/reload, still exists in the Autonomous System. For
more information see Section 13.4.

12.1.7. LS checksum

This field is the checksum of the complete contents of the
LSA, excepting the LS age field. The LS age field is
excepted so that an LSA's age can be incremented without
updating the checksum. The checksum used is the same that
is used for ISO connectionless datagrams; it is commonly
referred to as the Fletcher checksum. It is documented in
Annex B of [Ref6]. The LSA header also contains the length
of the LSA in bytes; subtracting the size of the LS age
field (two bytes) yields the amount of data to checksum.

The checksum is used to detect data corruption of an LSA.
This corruption can occur while an LSA is being flooded, or
while it is being held in a router's memory. The LS
checksum field cannot take on the value of zero; the
occurrence of such a value should be considered a checksum
failure. In other words, calculation of the checksum is not
optional.

The checksum of an LSA is verified in two cases: a) when it
is received in a Link State Update Packet and b) at times
during the aging of the link state database. The detection
of a checksum failure leads to separate actions in each
case. See Sections 13 and 14 for more details.

Whenever the LS sequence number field indicates that two
instances of an LSA are the same, the LS checksum field is
examined. If there is a difference, the instance with the
larger LS checksum is considered to be most recent.[13] See
Section 13.1 for more details.

12.2. The link state database

A router has a separate link state database for every area to
which it belongs. All routers belonging to the same area have
identical link state databases for the area.

The databases for each individual area are always dealt with
separately. The shortest path calculation is performed
separately for each area (see Section 16). Components of the

area link-state database are flooded throughout the area only.
Finally, when an adjacency (belonging to Area A) is being
brought up, only the database for Area A is synchronized between
the two routers.

The area database is composed of router-LSAs, network-LSAs and
summary-LSAs (all listed in the area data structure). In
addition, external routes (AS-external-LSAs) are included in all
non-stub area databases (see Section 3.6).

An implementation of OSPF must be able to access individual
pieces of an area database. This lookup function is based on an
LSA's LS type, Link State ID and Advertising Router.[14] There
will be a single instance (the most up-to-date) of each LSA in
the database. The database lookup function is invoked during
the LSA flooding procedure (Section 13) and the routing table
calculation (Section 16). In addition, using this lookup
function the router can determine whether it has itself ever
originated a particular LSA, and if so, with what LS sequence
number.

An LSA is added to a router's database when either a) it is
received during the flooding process (Section 13) or b) it is
originated by the router itself (Section 12.4). An LSA is
deleted from a router's database when either a) it has been
overwritten by a newer instance during the flooding process
(Section 13) or b) the router originates a newer instance of one
of its self-originated LSAs (Section 12.4) or c) the LSA ages
out and is flushed from the routing domain (Section 14).
Whenever an LSA is deleted from the database it must also be
removed from all neighbors' Link state retransmission lists (see
Section 10).

12.3. Representation of TOS

For backward compatibility with previous versions of the OSPF
specification ([Ref9]), TOS-specific information can be included
in router-LSAs, summary-LSAs and AS-external-LSAs. The encoding
of TOS in OSPF LSAs is specified in Table 17. That table relates
the OSPF encoding to the IP packet header's TOS field (defined
in [Ref12]). The OSPF encoding is expressed as a decimal

integer, and the IP packet header's TOS field is expressed in
the binary TOS values used in [Ref12].

OSPF encoding RFC1349 TOS values
___________________________________________
0 0000 normal service
2 0001 minimize monetary cost
4 0010 maximize reliability
6 0011
8 0100 maximize throughput
10 0101
12 0110
14 0111
16 1000 minimize delay
18 1001
20 1010
22 1011
24 1100
26 1101
28 1110
30 1111

Table 17: Representing TOS in OSPF.

12.4. Originating LSAs

Into any given OSPF area, a router will originate several LSAs.
Each router originates a router-LSA. If the router is also the
Designated Router for any of the area's networks, it will
originate network-LSAs for those networks.

Area border routers originate a single summary-LSA for each
known inter-area destination. AS boundary routers originate a
single AS-external-LSA for each known AS external destination.
Destinations are advertised one at a time so that the change in
any single route can be flooded without reflooding the entire
collection of routes. During the flooding procedure, many LSAs
can be carried by a single Link State Update packet.

As an example, consider Router RT4 in Figure 6. It is an area
border router, having a connection to Area 1 and the backbone.
Router RT4 originates 5 distinct LSAs into the backbone (one
router-LSA, and one summary-LSA for each of the networks N1-N4).
Router RT4 will also originate 8 distinct LSAs into Area 1 (one
router-LSA and seven summary-LSAs as pictured in Figure 7). If
RT4 has been selected as Designated Router for Network N3, it
will also originate a network-LSA for N3 into Area 1.

In this same figure, Router RT5 will be originating 3 distinct
AS-external-LSAs (one for each of the networks N12-N14). These
will be flooded throughout the entire AS, assuming that none of
the areas have been configured as stubs. However, if area 3 has
been configured as a stub area, the AS-external-LSAs for
networks N12-N14 will not be flooded into area 3 (see Section
3.6). Instead, Router RT11 would originate a default summary-
LSA that would be flooded throughout area 3 (see Section
12.4.3). This instructs all of area 3's internal routers to
send their AS external traffic to RT11.

Whenever a new instance of an LSA is originated, its LS sequence
number is incremented, its LS age is set to 0, its LS checksum
is calculated, and the LSA is added to the link state database
and flooded out the appropriate interfaces. See Section 13.2
for details concerning the installation of the LSA into the link
state database. See Section 13.3 for details concerning the
flooding of newly originated LSAs.

The ten events that can cause a new instance of an LSA to be
originated are:

(1) The LS age field of one of the router's self-originated LSAs
reaches the value LSRefreshTime. In this case, a new
instance of the LSA is originated, even though the contents
of the LSA (apart from the LSA header) will be the same.
This guarantees periodic originations of all LSAs. This
periodic updating of LSAs adds robustness to the link state
algorithm. LSAs that solely describe unreachable
destinations should not be refreshed, but should instead be
flushed from the routing domain (see Section 14.1).

When whatever is being described by an LSA changes, a new LSA is
originated. However, two instances of the same LSA may not be
originated within the time period MinLSInterval. This may
require that the generation of the next instance be delayed by
up to MinLSInterval. The following events may cause the
contents of an LSA to change. These events should cause new
originations if and only if the contents of the new LSA would be
different:

(2) An interface's state changes (see Section 9.1). This may
mean that it is necessary to produce a new instance of the
router-LSA.

(3) An attached network's Designated Router changes. A new
router-LSA should be originated. Also, if the router itself
is now the Designated Router, a new network-LSA should be
produced. If the router itself is no longer the Designated
Router, any network-LSA that it might have originated for
the network should be flushed from the routing domain (see
Section 14.1).

(4) One of the neighboring routers changes to/from the FULL
state. This may mean that it is necessary to produce a new
instance of the router-LSA. Also, if the router is itself
the Designated Router for the attached network, a new
network-LSA should be produced.

The next four events concern area border routers only:

(5) An intra-area route has been added/deleted/modified in the
routing table. This may cause a new instance of a summary-
LSA (for this route) to be originated in each attached area
(possibly including the backbone).

(6) An inter-area route has been added/deleted/modified in the
routing table. This may cause a new instance of a summary-
LSA (for this route) to be originated in each attached area
(but NEVER for the backbone).

(7) The router becomes newly attached to an area. The router
must then originate summary-LSAs into the newly attached
area for all pertinent intra-area and inter-area routes in
the router's routing table. See Section 12.4.3 for more
details.

(8) When the state of one of the router's configured virtual
links changes, it may be necessary to originate a new
router-LSA into the virtual link's Transit area (see the
discussion of the router-LSA's bit V in Section 12.4.1), as
well as originating a new router-LSA into the backbone.

The last two events concern AS boundary routers (and former AS
boundary routers) only:

(9) An external route gained through direct experience with an
external routing protocol (like BGP) changes. This will
cause an AS boundary router to originate a new instance of
an AS-external-LSA.

(10)
A router ceases to be an AS boundary router, perhaps after
restarting. In this situation the router should flush all
AS-external-LSAs that it had previously originated. These
LSAs can be flushed via the premature aging procedure
specified in Section 14.1.

The construction of each type of LSA is explained in detail
below. In general, these sections describe the contents of the
LSA body (i.e., the part coming after the 20-byte LSA header).
For information concerning the building of the LSA header, see
Section 12.1.

12.4.1. Router-LSAs

A router originates a router-LSA for each area that it
belongs to. Such an LSA describes the collected states of
the router's links to the area. The LSA is flooded
throughout the particular area, and no further.

....................................
. 192.1.2 Area 1 .
. + .
. .
. 3+---+1 .
. N1 --RT1-----+ .
. +---+ .
. _______N3 .
. + / . 1+---+
. * 192.1.1 *------RT4
. + /_______/ . +---+
. / .
. 3+---+1 / .
. N2 --RT2-----+ 1 .
. +---+ +---+8 . 6+---+
. RT3----------------RT6
. + +---+ . +---+
. 192.1.3 2 . 18.10.0.67
. .
. +------------+ .
. 192.1.4 (N4) .
....................................

Figure 15: Area 1 with IP addresses shown

The format of a router-LSA is shown in Appendix A (Section
A.4.2). The first 20 bytes of the LSA consist of the
generic LSA header that was discussed in Section 12.1.
router-LSAs have LS type = 1.

A router also indicates whether it is an area border router,
or an AS boundary router, by setting the appropriate bits
(bit B and bit E, respectively) in its router-LSAs. This
enables paths to those types of routers to be saved in the
routing table, for later processing of summary-LSAs and AS-
external-LSAs. Bit B should be set whenever the router is
actively attached to two or more areas, even if the router
is not currently attached to the OSPF backbone area. Bit E
should never be set in a router-LSA for a stub area (stub
areas cannot contain AS boundary routers).

In addition, the router sets bit V in its router-LSA for
Area A if and only if the router is the endpoint of one or
more fully adjacent virtual links having Area A as their
Transit area. The setting of bit V enables other routers in
Area A to discover whether the area supports transit traffic
(see TransitCapability in Section 6).

The router-LSA then describes the router's working
connections (i.e., interfaces or links) to the area. Each
link is typed according to the kind of attached network.
Each link is also labelled with its Link ID. This Link ID
gives a name to the entity that is on the other end of the
link. Table 18 summarizes the values used for the Type and
Link ID fields.

Link type Description Link ID
__________________________________________________
1 Point-to-point Neighbor Router ID
link
2 Link to transit Interface address of
network Designated Router
3 Link to stub IP network number
network
4 Virtual link Neighbor Router ID

Table 18: Link descriptions in the
router-LSA.

In addition, the Link Data field is specified for each link.
This field gives 32 bits of extra information for the link.
For links to transit networks, numbered point-to-point links
and virtual links, this field specifies the IP interface
address of the associated router interface (this is needed
by the routing table calculation, see Section 16.1.1). For
links to stub networks, this field specifies the stub
network's IP address mask. For unnumbered point-to-point
links, the Link Data field should be set to the unnumbered
interface's MIB-II [Ref8] ifIndex value.

Finally, the cost of using the link for output is specified.
The output cost of a link is configurable. With the
exception of links to stub networks, the output cost must
always be non-zero.

To further describe the process of building the list of link
descriptions, suppose a router wishes to build a router-LSA
for Area A. The router examines its collection of interface
data structures. For each interface, the following steps
are taken:

o If the attached network does not belong to Area A, no
links are added to the LSA, and the next interface
should be examined.

o If the state of the interface is Down, no links are
added.

o If the state of the interface is Loopback, add a Type 3
link (stub network) as long as this is not an interface
to an unnumbered point-to-point network. The Link ID
should be set to the IP interface address, the Link Data
set to the mask 0xffffffff (indicating a host route),
and the cost set to 0.

o Otherwise, the link descriptions added to the router-LSA
depend on the OSPF interface type. Link descriptions
used for point-to-point interfaces are specified in
Section 12.4.1.1, for virtual links in Section 12.4.1.2,
for broadcast and NBMA interfaces in 12.4.1.3, and for
Point-to-MultiPoint interfaces in 12.4.1.4.

After consideration of all the router interfaces, host links
are added to the router-LSA by examining the list of
attached hosts belonging to Area A. A host route is
represented as a Type 3 link (stub network) whose Link ID is
the host's IP address, Link Data is the mask of all ones
(0xffffffff), and cost the host's configured cost (see
Section C.7).

12.4.1.1. Describing point-to-point interfaces

For point-to-point interfaces, one or more link
descriptions are added to the router-LSA as follows:

o If the neighboring router is fully adjacent, add a
Type 1 link (point-to-point). The Link ID should be
set to the Router ID of the neighboring router. For
numbered point-to-point networks, the Link Data
should specify the IP interface address. For
unnumbered point-to-point networks, the Link Data
field should specify the interface's MIB-II [Ref8]
ifIndex value. The cost should be set to the output
cost of the point-to-point interface.

o In addition, as long as the state of the interface
is "Point-to-Point" (and regardless of the
neighboring router state), a Type 3 link (stub
network) should be added. There are two forms that
this stub link can take:

Option 1
Assuming that the neighboring router's IP
address is known, set the Link ID of the Type 3
link to the neighbor's IP address, the Link Data
to the mask 0xffffffff (indicating a host
route), and the cost to the interface's
configured output cost.[15]

Option 2
If a subnet has been assigned to the point-to-
point link, set the Link ID of the Type 3 link
to the subnet's IP address, the Link Data to the
subnet's mask, and the cost to the interface's
configured output cost.[16]

12.4.1.2. Describing broadcast and NBMA interfaces

For operational broadcast and NBMA interfaces, a single
link description is added to the router-LSA as follows:

o If the state of the interface is Waiting, add a Type
3 link (stub network) with Link ID set to the IP
network number of the attached network, Link Data
set to the attached network's address mask, and cost
equal to the interface's configured output cost.

o Else, there has been a Designated Router elected for
the attached network. If the router is fully
adjacent to the Designated Router, or if the router
itself is Designated Router and is fully adjacent to
at least one other router, add a single Type 2 link
(transit network) with Link ID set to the IP
interface address of the attached network's
Designated Router (which may be the router itself),
Link Data set to the router's own IP interface
address, and cost equal to the interface's
configured output cost. Otherwise, add a link as if
the interface state were Waiting (see above).

12.4.1.3. Describing virtual links

For virtual links, a link description is added to the
router-LSA only when the virtual neighbor is fully
adjacent. In this case, add a Type 4 link (virtual link)
with Link ID set to the Router ID of the virtual
neighbor, Link Data set to the IP interface address
associated with the virtual link and cost set to the
cost calculated for the virtual link during the routing
table calculation (see Section 15).

12.4.1.4. Describing Point-to-MultiPoint interfaces

For operational Point-to-MultiPoint interfaces, one or
more link descriptions are added to the router-LSA as
follows:

o A single Type 3 link (stub network) is added with
Link ID set to the router's own IP interface
address, Link Data set to the mask 0xffffffff
(indicating a host route), and cost set to 0.

o For each fully adjacent neighbor associated with the
interface, add an additional Type 1 link (point-to-
point) with Link ID set to the Router ID of the
neighboring router, Link Data set to the IP
interface address and cost equal to the interface's
configured output cost.

12.4.1.5. Examples of router-LSAs

Consider the router-LSAs generated by Router RT3, as
pictured in Figure 6. The area containing Router RT3
(Area 1) has been redrawn, with actual network
addresses, in Figure 15. Assume that the last byte of
all of RT3's interface addresses is 3, giving it the
interface addresses 192.1.1.3 and 192.1.4.3, and that
the other routers have similar addressing schemes. In
addition, assume that all links are functional, and that
Router IDs are assigned as the smallest IP interface
address.

RT3 originates two router-LSAs, one for Area 1 and one
for the backbone. Assume that Router RT4 has been
selected as the Designated router for network 192.1.1.0.
RT3's router-LSA for Area 1 is then shown below. It
indicates that RT3 has two connections to Area 1, the
first a link to the transit network 192.1.1.0 and the
second a link to the stub network 192.1.4.0. Note that
the transit network is identified by the IP interface of
its Designated Router (i.e., the Link ID = 192.1.1.4
which is the Designated Router RT4's IP interface to
192.1.1.0). Note also that RT3 has indicated that it is
an area border router.

; RT3's router-LSA for Area 1

LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 1 ;indicates router-LSA
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router

bit B = 1 ;area border router
#links = 2
Link ID = 192.1.1.4 ;IP address of Desig. Rtr.
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# TOS metrics = 0
metric = 1

Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network
# TOS metrics = 0
metric = 2

Next RT3's router-LSA for the backbone is shown. It
indicates that RT3 has a single attachment to the
backbone. This attachment is via an unnumbered
point-to-point link to Router RT6. RT3 has again
indicated that it is an area border router.

; RT3's router-LSA for the backbone

LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 1 ;indicates router-LSA
Link State ID = 192.1.1.3 ;RT3's router ID
Advertising Router = 192.1.1.3 ;RT3's router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# TOS metrics = 0
metric = 8

12.4.2. Network-LSAs

A network-LSA is generated for every transit broadcast or
NBMA network. (A transit network is a network having two or
more attached routers). The network-LSA describes all the
routers that are attached to the network.

The Designated Router for the network originates the LSA.
The Designated Router originates the LSA only if it is fully
adjacent to at least one other router on the network. The
network-LSA is flooded throughout the area that contains the
transit network, and no further. The network-LSA lists
those routers that are fully adjacent to the Designated
Router; each fully adjacent router is identified by its OSPF
Router ID. The Designated Router includes itself in this
list.

The Link State ID for a network-LSA is the IP interface
address of the Designated Router. This value, masked by the
network's address mask (which is also contained in the
network-LSA) yields the network's IP address.

A router that has formerly been the Designated Router for a
network, but is no longer, should flush the network-LSA that
it had previously originated. This LSA is no longer used in
the routing table calculation. It is flushed by prematurely
incrementing the LSA's age to MaxAge and reflooding (see
Section 14.1). In addition, in those rare cases where a
router's Router ID has changed, any network-LSAs that were
originated with the router's previous Router ID must be
flushed. Since the router may have no idea what it's
previous Router ID might have been, these network-LSAs are
indicated by having their Link State ID equal to one of the
router's IP interface addresses and their Advertising Router
equal to some value other than the router's current Router
ID (see Section 13.4 for more details).

12.4.2.1. Examples of network-LSAs

Again consider the area configuration in Figure 6.
Network-LSAs are originated for Network N3 in Area 1,
Networks N6 and N8 in Area 2, and Network N9 in Area 3.
Assuming that Router RT4 has been selected as the
Designated Router for Network N3, the following
network-LSA is generated by RT4 on behalf of Network N3
(see Figure 15 for the address assignments):

; Network-LSA for Network N3

LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 2 ;indicates network-LSA
Link State ID = 192.1.1.4 ;IP address of Desig. Rtr.
Advertising Router = 192.1.1.4 ;RT4's Router ID
Network Mask = 0xffffff00
Attached Router = 192.1.1.4 ;Router ID
Attached Router = 192.1.1.1 ;Router ID
Attached Router = 192.1.1.2 ;Router ID
Attached Router = 192.1.1.3 ;Router ID

12.4.3. Summary-LSAs

The destination described by a summary-LSA is either an IP
network, an AS boundary router or a range of IP addresses.
Summary-LSAs are flooded throughout a single area only. The
destination described is one that is external to the area,
yet still belongs to the Autonomous System.

Summary-LSAs are originated by area border routers. The
precise summary routes to advertise into an area are
determined by examining the routing table structure (see
Section 11) in accordance with the algorithm described
below. Note that only intra-area routes are advertised into
the backbone, while both intra-area and inter-area routes
are advertised into the other areas.

To determine which routes to advertise into an attached Area
A, each routing table entry is processed as follows.
Remember that each routing table entry describes a set of
equal-cost best paths to a particular destination:

o Only Destination Types of network and AS boundary router
are advertised in summary-LSAs. If the routing table
entry's Destination Type is area border router, examine
the next routing table entry.

o AS external routes are never advertised in summary-LSAs.
If the routing table entry has Path-type of type 1
external or type 2 external, examine the next routing
table entry.

o Else, if the area associated with this set of paths is
the Area A itself, do not generate a summary-LSA for the
route.[17]

o Else, if the next hops associated with this set of paths
belong to Area A itself, do not generate a summary-LSA
for the route.[18] This is the logical equivalent of a
Distance Vector protocol's split horizon logic.

o Else, if the routing table cost equals or exceeds the
value LSInfinity, a summary-LSA cannot be generated for
this route.

o Else, if the destination of this route is an AS boundary
router, a summary-LSA should be originated if and only
if the routing table entry describes the preferred path
to the AS boundary router (see Step 3 of Section 16.4).
If so, a Type 4 summary-LSA is originated for the
destination, with Link State ID equal to the AS boundary
router's Router ID and metric equal to the routing table
entry's cost. Note: these LSAs should not be generated
if Area A has been configured as a stub area.

o Else, the Destination type is network. If this is an
inter-area route, generate a Type 3 summary-LSA for the
destination, with Link State ID equal to the network's
address (if necessary, the Link State ID can also have
one or more of the network's host bits set; see Appendix
E for details) and metric equal to the routing table
cost.

o The one remaining case is an intra-area route to a
network. This means that the network is contained in
one of the router's directly attached areas. In
general, this information must be condensed before
appearing in summary-LSAs. Remember that an area has a
configured list of address ranges, each range consisting
of an [address,mask] pair and a status indication of
either Advertise or DoNotAdvertise. At most a single
Type 3 summary-LSA is originated for each range. When
the range's status indicates Advertise, a Type 3
summary-LSA is generated with Link State ID equal to the

range's address (if necessary, the Link State ID can
also have one or more of the range's "host" bits set;
see Appendix E for details) and cost equal to the
largest cost of any of the component networks. When the
range's status indicates DoNotAdvertise, the Type 3
summary-LSA is suppressed and the component networks
remain hidden from other areas.

By default, if a network is not contained in any
explicitly configured address range, a Type 3 summary-
LSA is generated with Link State ID equal to the
network's address (if necessary, the Link State ID can
also have one or more of the network's "host" bits set;
see Appendix E for details) and metric equal to the
network's routing table cost.

If an area is capable of carrying transit traffic (i.e.,
its TransitCapability is set to TRUE), routing
information concerning backbone networks should not be
condensed before being summarized into the area. Nor
should the advertisement of backbone networks into
transit areas be suppressed. In other words, the
backbone's configured ranges should be ignored when
originating summary-LSAs into transit areas.

If a router advertises a summary-LSA for a destination which
then becomes unreachable, the router must then flush the LSA
from the routing domain by setting its age to MaxAge and
reflooding (see Section 14.1). Also, if the destination is
still reachable, yet can no longer be advertised according
to the above procedure (e.g., it is now an inter-area route,
when it used to be an intra-area route associated with some
non-backbone area; it would thus no longer be advertisable
to the backbone), the LSA should also be flushed from the
routing domain.

12.4.3.1. Originating summary-LSAs into stub areas

The algorithm in Section 12.4.3 is optional when Area A
is an OSPF stub area. Area border routers connecting to
a stub area can originate summary-LSAs into the area

according to the Section 12.4.3's algorithm, or can
choose to originate only a subset of the summary-LSAs,
possibly under configuration control. The fewer LSAs
originated, the smaller the stub area's link state
database, further reducing the demands on its routers'
resources. However, omitting LSAs may also lead to sub-
optimal inter-area routing, although routing will
continue to function.

As specified in Section 12.4.3, Type 4 summary-LSAs
(ASBR-summary-LSAs) are never originated into stub
areas.

In a stub area, instead of importing external routes
each area border router originates a "default summary-
LSA" into the area. The Link State ID for the default
summary-LSA is set to DefaultDestination, and the metric
set to the (per-area) configurable parameter
StubDefaultCost. Note that StubDefaultCost need not be
configured identically in all of the stub area's area
border routers.

12.4.3.2. Examples of summary-LSAs

Consider again the area configuration in Figure 6.
Routers RT3, RT4, RT7, RT10 and RT11 are all area border
routers, and therefore are originating summary-LSAs.
Consider in particular Router RT4. Its routing table
was calculated as the example in Section 11.3. RT4
originates summary-LSAs into both the backbone and Area
1. Into the backbone, Router RT4 originates separate
LSAs for each of the networks N1-N4. Into Area 1,
Router RT4 originates separate LSAs for networks N6-N8
and the AS boundary routers RT5,RT7. It also condenses
host routes Ia and Ib into a single summary-LSA.
Finally, the routes to networks N9,N10,N11 and Host H1
are advertised by a single summary-LSA. This
condensation was originally performed by the router
RT11.

These LSAs are illustrated graphically in Figures 7 and
8. Two of the summary-LSAs originated by Router RT4
follow. The actual IP addresses for the networks and
routers in question have been assigned in Figure 15.

; Summary-LSA for Network N1,
; originated by Router RT4 into the backbone

LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 3 ;Type 3 summary-LSA
Link State ID = 192.1.2.0 ;N1's IP network number
Advertising Router = 192.1.1.4 ;RT4's ID
metric = 4

; Summary-LSA for AS boundary router RT7
; originated by Router RT4 into Area 1

LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 4 ;Type 4 summary-LSA
Link State ID = Router RT7's ID
Advertising Router = 192.1.1.4 ;RT4's ID
metric = 14

12.4.4. AS-external-LSAs

AS-external-LSAs describe routes to destinations external to
the Autonomous System. Most AS-external-LSAs describe
routes to specific external destinations; in these cases the
LSA's Link State ID is set to the destination network's IP
address (if necessary, the Link State ID can also have one
or more of the network's "host" bits set; see Appendix E for
details). However, a default route for the Autonomous
System can be described in an AS-external-LSA by setting the
LSA's Link State ID to DefaultDestination (0.0.0.0). AS-
external-LSAs are originated by AS boundary routers. An AS
boundary router originates a single AS-external-LSA for each
external route that it has learned, either through another
routing protocol (such as BGP), or through configuration
information.

AS-external-LSAs are the only type of LSAs that are flooded
throughout the entire Autonomous System; all other types of
LSAs are specific to a single area. However, AS-external-
LSAs are not flooded into/throughout stub areas (see Section
3.6). This enables a reduction in link state database size
for routers internal to stub areas.

The metric that is advertised for an external route can be
one of two types. Type 1 metrics are comparable to the link
state metric. Type 2 metrics are assumed to be larger than
the cost of any intra-AS path.

If a router advertises an AS-external-LSA for a destination
which then becomes unreachable, the router must then flush
the LSA from the routing domain by setting its age to MaxAge
and reflooding (see Section 14.1).

12.4.4.1. Examples of AS-external-LSAs

Consider once again the AS pictured in Figure 6. There
are two AS boundary routers: RT5 and RT7. Router RT5
originates three AS-external-LSAs, for networks N12-N14.
Router RT7 originates two AS-external-LSAs, for networks
N12 and N15. Assume that RT7 has learned its route to
N12 via BGP, and that it wishes to advertise a Type 2
metric to the AS. RT7 would then originate the
following LSA for N12:

; AS-external-LSA for Network N12,
; originated by Router RT7

LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 5 ;AS-external-LSA
Link State ID = N12's IP network number
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
metric = 2
Forwarding address = 0.0.0.0

In the above example, the forwarding address field
has been set to 0.0.0.0, indicating that packets for
the external destination should be forwarded to the
advertising OSPF router (RT7). This is not always
desirable. Consider the example pictured in Figure
16. There are three OSPF routers (RTA, RTB and RTC)
connected to a common network. Only one of these
routers, RTA, is exchanging BGP information with the
non-OSPF router RTX. RTA must then originate AS-
external-LSAs for those destinations it has learned
from RTX. By using the AS-external-LSA's forwarding
address field, RTA can specify that packets for
these destinations be forwarded directly to RTX.
Without this feature, Routers RTB and RTC would take
an extra hop to get to these destinations.

Note that when the forwarding address field is non-
zero, it should point to a router belonging to
another Autonomous System.

A forwarding address can also be specified for the
default route. For example, in figure 16 RTA may
want to specify that all externally-destined packets
should by default be forwarded to its BGP peer RTX.
The resulting AS-external-LSA is pictured below.
Note that the Link State ID is set to
DefaultDestination.

; Default route, originated by Router RTA
; Packets forwarded through RTX

LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 5 ;AS-external-LSA
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
metric = 1
Forwarding address = RTX's IP address

In figure 16, suppose instead that both RTA and RTB
exchange BGP information with RTX. In this case,

RTA and RTB would originate the same set of AS-
external-LSAs. These LSAs, if they specify the same
metric, would be functionally equivalent since they
would specify the same destination and forwarding
address (RTX). This leads to a clear duplication of
effort. If only one of RTA or RTB originated the
set of AS-external-LSAs, the routing would remain
the same, and the size of the link state database
would decrease. However, it must be unambiguously
defined as to which router originates the LSAs
(otherwise neither may, or the identity of the
originator may oscillate). The following rule is
thereby established: if two routers, both reachable
from one another, originate functionally equivalent
AS-external-LSAs (i.e., same destination, cost and
non-zero forwarding address), then the LSA
originated by the router having the highest OSPF
Router ID is used. The router having the lower OSPF
Router ID can then flush its LSA. Flushing an LSA
is discussed in Section 14.1.

+

+---+......BGP
RTA-----.....+---+
+---+ -----RTX
+---+
+---+
RTB-----
+---+

+---+
RTC-----
+---+

+

Figure 16: Forwarding address example

13. The Flooding Procedure

Link State Update packets provide the mechanism for flooding LSAs.
A Link State Update packet may contain several distinct LSAs, and
floods each LSA one hop further from its point of origination. To
make the flooding procedure reliable, each LSA must be acknowledged
separately. Acknowledgments are transmitted in Link State
Acknowledgment packets. Many separate acknowledgments can also be
grouped together into a single packet.

The flooding procedure starts when a Link State Update packet has
been received. Many consistency checks have been made on the
received packet before being handed to the flooding procedure (see
Section 8.2). In particular, the Link State Update packet has been
associated with a particular neighbor, and a particular area. If
the neighbor is in a lesser state than Exchange, the packet should
be dropped without further processing.

All types of LSAs, other than AS-external-LSAs, are associated with
a specific area. However, LSAs do not contain an area field. An
LSA's area must be deduced from the Link State Update packet header.

For each LSA contained in a Link State Update packet, the following
steps are taken:

(1) Validate the LSA's LS checksum. If the checksum turns out to be
invalid, discard the LSA and get the next one from the Link
State Update packet.

(2) Examine the LSA's LS type. If the LS type is unknown, discard
the LSA and get the next one from the Link State Update Packet.
This specification defines LS types 1-5 (see Section 4.3).

(3) Else if this is an AS-external-LSA (LS type = 5), and the area
has been configured as a stub area, discard the LSA and get the
next one from the Link State Update Packet. AS-external-LSAs
are not flooded into/throughout stub areas (see Section 3.6).

(4) Else if the LSA's LS age is equal to MaxAge, and there is
currently no instance of the LSA in the router's link state
database, and none of router's neighbors are in states Exchange

or Loading, then take the following actions: a) Acknowledge the
receipt of the LSA by sending a Link State Acknowledgment packet
back to the sending neighbor (see Section 13.5), and b) Discard
the LSA and examine the next LSA (if any) listed in the Link
State Update packet.

(5) Otherwise, find the instance of this LSA that is currently
contained in the router's link state database. If there is no
database copy, or the received LSA is more recent than the
database copy (see Section 13.1 below for the determination of
which LSA is more recent) the following steps must be performed:

(a) If there is already a database copy, and if the database
copy was received via flooding and installed less than
MinLSArrival seconds ago, discard the new LSA (without
acknowledging it) and examine the next LSA (if any) listed
in the Link State Update packet.

(b) Otherwise immediately flood the new LSA out some subset of
the router's interfaces (see Section 13.3). In some cases
(e.g., the state of the receiving interface is DR and the
LSA was received from a router other than the Backup DR) the
LSA will be flooded back out the receiving interface. This
occurrence should be noted for later use by the
acknowledgment process (Section 13.5).

(c) Remove the current database copy from all neighbors' Link
state retransmission lists.

(d) Install the new LSA in the link state database (replacing
the current database copy). This may cause the routing
table calculation to be scheduled. In addition, timestamp
the new LSA with the current time (i.e., the time it was
received). The flooding procedure cannot overwrite the
newly installed LSA until MinLSArrival seconds have elapsed.
The LSA installation process is discussed further in Section
13.2.

(e) Possibly acknowledge the receipt of the LSA by sending a
Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.

(f) If this new LSA indicates that it was originated by the
receiving router itself (i.e., is considered a self-
originated LSA), the router must take special action, either
updating the LSA or in some cases flushing it from the
routing domain. For a description of how self-originated
LSAs are detected and subsequently handled, see Section
13.4.

(6) Else, if there is an instance of the LSA on the sending
neighbor's Link state request list, an error has occurred in the
Database Exchange process. In this case, restart the Database
Exchange process by generating the neighbor event BadLSReq for
the sending neighbor and stop processing the Link State Update
packet.

(7) Else, if the received LSA is the same instance as the database
copy (i.e., neither one is more recent) the following two steps
should be performed:

(a) If the LSA is listed in the Link state retransmission list
for the receiving adjacency, the router itself is expecting
an acknowledgment for this LSA. The router should treat the
received LSA as an acknowledgment by removing the LSA from
the Link state retransmission list. This is termed an
"implied acknowledgment". Its occurrence should be noted
for later use by the acknowledgment process (Section 13.5).

(b) Possibly acknowledge the receipt of the LSA by sending a
Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.

(8) Else, the database copy is more recent. If the database copy
has LS age equal to MaxAge and LS sequence number equal to
MaxSequenceNumber, simply discard the received LSA without
acknowledging it. (In this case, the LSA's LS sequence number is
wrapping, and the MaxSequenceNumber LSA must be completely
flushed before any new LSA instance can be introduced).
Otherwise, as long as the database copy has not been sent in a
Link State Update within the last MinLSArrival seconds, send the
database copy back to the sending neighbor, encapsulated within
a Link State Update Packet. The Link State Update Packet should
be sent directly to the neighbor. In so doing, do not put the

database copy of the LSA on the neighbor's link state
retransmission list, and do not acknowledge the received (less
recent) LSA instance.

13.1. Determining which LSA is newer

When a router encounters two instances of an LSA, it must
determine which is more recent. This occurred above when
comparing a received LSA to its database copy. This comparison
must also be done during the Database Exchange procedure which
occurs during adjacency bring-up.

An LSA is identified by its LS type, Link State ID and
Advertising Router. For two instances of the same LSA, the LS
sequence number, LS age, and LS checksum fields are used to
determine which instance is more recent:

o The LSA having the newer LS sequence number is more recent.
See Section 12.1.6 for an explanation of the LS sequence
number space. If both instances have the same LS sequence
number, then:

o If the two instances have different LS checksums, then the
instance having the larger LS checksum (when considered as a
16-bit unsigned integer) is considered more recent.

o Else, if only one of the instances has its LS age field set
to MaxAge, the instance of age MaxAge is considered to be
more recent.

o Else, if the LS age fields of the two instances differ by
more than MaxAgeDiff, the instance having the smaller
(younger) LS age is considered to be more recent.

o Else, the two instances are considered to be identical.

13.2. Installing LSAs in the database

Installing a new LSA in the database, either as the result of
flooding or a newly self-originated LSA, may cause the OSPF
routing table structure to be recalculated. The contents of the
new LSA should be compared to the old instance, if present. If
there is no difference, there is no need to recalculate the
routing table. When comparing an LSA to its previous instance,
the following are all considered to be differences in contents:

o The LSA's Options field has changed.

o One of the LSA instances has LS age set to MaxAge, and
the other does not.

o The length field in the LSA header has changed.

o The body of the LSA (i.e., anything outside the 20-byte
LSA header) has changed. Note that this excludes changes
in LS Sequence Number and LS Checksum.

If the contents are different, the following pieces of the
routing table must be recalculated, depending on the new LSA's
LS type field:

Router-LSAs and network-LSAs
The entire routing table must be recalculated, starting with
the shortest path calculations for each area (not just the
area whose link-state database has changed). The reason
that the shortest path calculation cannot be restricted to
the single changed area has to do with the fact that AS
boundary routers may belong to multiple areas. A change in
the area currently providing the best route may force the
router to use an intra-area route provided by a different
area.[19]

Summary-LSAs
The best route to the destination described by the summary-
LSA must be recalculated (see Section 16.5). If this
destination is an AS boundary router, it may also be
necessary to re-examine all the AS-external-LSAs.

AS-external-LSAs
The best route to the destination described by the AS-
external-LSA must be recalculated (see Section 16.6).

Also, any old instance of the LSA must be removed from the
database when the new LSA is installed. This old instance must
also be removed from all neighbors' Link state retransmission
lists (see Section 10).

13.3. Next step in the flooding procedure

When a new (and more recent) LSA has been received, it must be
flooded out some set of the router's interfaces. This section
describes the second part of flooding procedure (the first part
being the processing that occurred in Section 13), namely,
selecting the outgoing interfaces and adding the LSA to the
appropriate neighbors' Link state retransmission lists. Also
included in this part of the flooding procedure is the
maintenance of the neighbors' Link state request lists.

This section is equally applicable to the flooding of an LSA
that the router itself has just originated (see Section 12.4).
For these LSAs, this section provides the entirety of the
flooding procedure (i.e., the processing of Section 13 is not
performed, since, for example, the LSA has not been received
from a neighbor and therefore does not need to be acknowledged).

Depending upon the LSA's LS type, the LSA can be flooded out
only certain interfaces. These interfaces, defined by the
following, are called the eligible interfaces:

AS-external-LSAs (LS Type = 5)
AS-external-LSAs are flooded throughout the entire AS, with
the exception of stub areas (see Section 3.6). The eligible
interfaces are all the router's interfaces, excluding
virtual links and those interfaces attaching to stub areas.

All other LS types
All other types are specific to a single area (Area A). The

eligible interfaces are all those interfaces attaching to
the Area A. If Area A is the backbone, this includes all
the virtual links.

Link state databases must remain synchronized over all
adjacencies associated with the above eligible interfaces. This
is accomplished by executing the following steps on each
eligible interface. It should be noted that this procedure may
decide not to flood an LSA out a particular interface, if there
is a high probability that the attached neighbors have already
received the LSA. However, in these cases the flooding
procedure must be absolutely sure that the neighbors eventually
do receive the LSA, so the LSA is still added to each
adjacency's Link state retransmission list. For each eligible
interface:

(1) Each of the neighbors attached to this interface are
examined, to determine whether they must receive the new
LSA. The following steps are executed for each neighbor:

(a) If the neighbor is in a lesser state than Exchange, it
does not participate in flooding, and the next neighbor
should be examined.

(b) Else, if the adjacency is not yet full (neighbor state
is Exchange or Loading), examine the Link state request
list associated with this adjacency. If there is an
instance of the new LSA on the list, it indicates that
the neighboring router has an instance of the LSA
already. Compare the new LSA to the neighbor's copy:

o If the new LSA is less recent, then examine the next
neighbor.

o If the two copies are the same instance, then delete
the LSA from the Link state request list, and
examine the next neighbor.[20]

o Else, the new LSA is more recent. Delete the LSA
from the Link state request list.

(c) If the new LSA was received from this neighbor, examine
the next neighbor.

(d) At this point we are not positive that the neighbor has
an up-to-date instance of this new LSA. Add the new LSA
to the Link state retransmission list for the adjacency.
This ensures that the flooding procedure is reliable;
the LSA will be retransmitted at intervals until an
acknowledgment is seen from the neighbor.

(2) The router must now decide whether to flood the new LSA out
this interface. If in the previous step, the LSA was NOT
added to any of the Link state retransmission lists, there
is no need to flood the LSA out the interface and the next
interface should be examined.

(3) If the new LSA was received on this interface, and it was
received from either the Designated Router or the Backup
Designated Router, chances are that all the neighbors have
received the LSA already. Therefore, examine the next
interface.

(4) If the new LSA was received on this interface, and the
interface state is Backup (i.e., the router itself is the
Backup Designated Router), examine the next interface. The
Designated Router will do the flooding on this interface.
However, if the Designated Router fails the router (i.e.,
the Backup Designated Router) will end up retransmitting the
updates.

(5) If this step is reached, the LSA must be flooded out the
interface. Send a Link State Update packet (including the
new LSA as contents) out the interface. The LSA's LS age
must be incremented by InfTransDelay (which must be > 0)
when it is copied into the outgoing Link State Update packet
(until the LS age field reaches the maximum value of
MaxAge).

On broadcast networks, the Link State Update packets are
multicast. The destination IP address specified for the
Link State Update Packet depends on the state of the
interface. If the interface state is DR or Backup, the

address AllSPFRouters should be used. Otherwise, the
address AllDRouters should be used.

On non-broadcast networks, separate Link State Update
packets must be sent, as unicasts, to each adjacent neighbor
(i.e., those in state Exchange or greater). The destination
IP addresses for these packets are the neighbors' IP
addresses.

13.4. Receiving self-originated LSAs

It is a common occurrence for a router to receive self-
originated LSAs via the flooding procedure. A self-originated
LSA is detected when either 1) the LSA's Advertising Router is
equal to the router's own Router ID or 2) the LSA is a network-
LSA and its Link State ID is equal to one of the router's own IP
interface addresses.

However, if the received self-originated LSA is newer than the
last instance that the router actually originated, the router
must take special action. The reception of such an LSA
indicates that there are LSAs in the routing domain that were
originated by the router before the last time it was restarted.
In most cases, the router must then advance the LSA's LS
sequence number one past the received LS sequence number, and
originate a new instance of the LSA.

It may be the case the router no longer wishes to originate the
received LSA. Possible examples include: 1) the LSA is a
summary-LSA or AS-external-LSA and the router no longer has an
(advertisable) route to the destination, 2) the LSA is a
network-LSA but the router is no longer Designated Router for
the network or 3) the LSA is a network-LSA whose Link State ID
is one of the router's own IP interface addresses but whose
Advertising Router is not equal to the router's own Router ID
(this latter case should be rare, and it indicates that the
router's Router ID has changed since originating the LSA). In
all these cases, instead of updating the LSA, the LSA should be
flushed from the routing domain by incrementing the received
LSA's LS age to MaxAge and reflooding (see Section 14.1).

13.5. Sending Link State Acknowledgment packets

Each newly received LSA must be acknowledged. This is usually
done by sending Link State Acknowledgment packets. However,
acknowledgments can also be accomplished implicitly by sending
Link State Update packets (see step 7a of Section 13).

Many acknowledgments may be grouped together into a single Link
State Acknowledgment packet. Such a packet is sent back out the
interface which received the LSAs. The packet can be sent in
one of two ways: delayed and sent on an interval timer, or sent
directly to a particular neighbor. The particular
acknowledgment strategy used depends on the circumstances
surrounding the receipt of the LSA.

Sending delayed acknowledgments accomplishes several things: 1)
it facilitates the packaging of multiple acknowledgments in a
single Link State Acknowledgment packet, 2) it enables a single
Link State Acknowledgment packet to indicate acknowledgments to
several neighbors at once (through multicasting) and 3) it
randomizes the Link State Acknowledgment packets sent by the
various routers attached to a common network. The fixed
interval between a router's delayed transmissions must be short
(less than RxmtInterval) or needless retransmissions will ensue.

Direct acknowledgments are sent directly to a particular
neighbor in response to the receipt of duplicate LSAs. Direct
acknowledgments are sent immediately when the duplicate is
received. On multi-access networks, these acknowledgments are
sent directly to the neighbor's IP address.

The precise procedure for sending Link State Acknowledgment
packets is described in Table 19. The circumstances surrounding
the receipt of the LSA are listed in the left column. The
acknowledgment action then taken is listed in one of the two
right columns. This action depends on the state of the
concerned interface; interfaces in state Backup behave
differently from interfaces in all other states. Delayed
acknowledgments must be delivered to all adjacent routers
associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment
packets as multicasts. The Destination IP address used depends

Action taken in state
Circumstances Backup All other states
_________________________________________________________________
LSA has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
_________________________________________________________________
LSA is Delayed acknowledg- Delayed ack-
more recent than ment sent if adver- nowledgment sent.
database copy, but tisement received
was not flooded from Designated
back out receiving Router, otherwise
interface do nothing
_________________________________________________________________
LSA is a Delayed acknowledg- No acknowledgment
duplicate, and was ment sent if adver- sent.
treated as an im- tisement received
plied acknowledg- from Designated
ment (see Section Router, otherwise
13, step 7a). do nothing
_________________________________________________________________
LSA is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
implied ack-
nowledgment.
_________________________________________________________________
LSA's LS Direct acknowledg- Direct acknowledg-
age is equal to ment sent. ment sent.
MaxAge, and there is
no current instance
of the LSA
in the link state
database, and none
of router's neighbors
are in states Exchange

or Loading (see
Section 13, step 4).

Table 19: Sending link state acknowledgements.

on the state of the interface. If the interface state is DR or
Backup, the destination AllSPFRouters is used. In all other
states, the destination AllDRouters is used. On non-broadcast
networks, delayed Link State Acknowledgment packets must be
unicast separately over each adjacency (i.e., neighbor whose
state is >= Exchange).

The reasoning behind sending the above packets as multicasts is
best explained by an example. Consider the network
configuration depicted in Figure 15. Suppose RT4 has been
elected as Designated Router, and RT3 as Backup Designated
Router for the network N3. When Router RT4 floods a new LSA to
Network N3, it is received by routers RT1, RT2, and RT3. These
routers will not flood the LSA back onto net N3, but they still
must ensure that their link-state databases remain synchronized
with their adjacent neighbors. So RT1, RT2, and RT4 are waiting
to see an acknowledgment from RT3. Likewise, RT4 and RT3 are
both waiting to see acknowledgments from RT1 and RT2. This is
best achieved by sending the acknowledgments as multicasts.

The reason that the acknowledgment logic for Backup DRs is
slightly different is because they perform differently during
the flooding of LSAs (see Section 13.3, step 4).

13.6. Retransmitting LSAs

LSAs flooded out an adjacency are placed on the adjacency's Link
state retransmission list. In order to ensure that flooding is
reliable, these LSAs are retransmitted until they are
acknowledged. The length of time between retransmissions is a
configurable per-interface value, RxmtInterval. If this is set

too low for an interface, needless retransmissions will ensue.
If the value is set too high, the speed of the flooding, in the
face of lost packets, may be affected.

Several retransmitted LSAs may fit into a single Link State
Update packet. When LSAs are to be retransmitted, only the
number fitting in a single Link State Update packet should be
sent. Another packet of retransmissions can be sent whenever
some of the LSAs are acknowledged, or on the next firing of the
retransmission timer.

Link State Update Packets carrying retransmissions are always
sent directly to the neighbor. On multi-access networks, this
means that retransmissions are sent directly to the neighbor's
IP address. Each LSA's LS age must be incremented by
InfTransDelay (which must be > 0) when it is copied into the
outgoing Link State Update packet (until the LS age field
reaches the maximum value of MaxAge).

If an adjacent router goes down, retransmissions may occur until
the adjacency is destroyed by OSPF's Hello Protocol. When the
adjacency is destroyed, the Link state retransmission list is
cleared.

13.7. Receiving link state acknowledgments

Many consistency checks have been made on a received Link State
Acknowledgment packet before it is handed to the flooding
procedure. In particular, it has been associated with a
particular neighbor. If this neighbor is in a lesser state than
Exchange, the Link State Acknowledgment packet is discarded.

Otherwise, for each acknowledgment in the Link State
Acknowledgment packet, the following steps are performed:

o Does the LSA acknowledged have an instance on the Link state
retransmission list for the neighbor? If not, examine the
next acknowledgment. Otherwise:

o If the acknowledgment is for the same instance that is
contained on the list, remove the item from the list and
examine the next acknowledgment. Otherwise:

o Log the questionable acknowledgment, and examine the next
one.

14. Aging The Link State Database

Each LSA has an LS age field. The LS age is expressed in seconds.
An LSA's LS age field is incremented while it is contained in a
router's database. Also, when copied into a Link State Update
Packet for flooding out a particular interface, the LSA's LS age is
incremented by InfTransDelay.

An LSA's LS age is never incremented past the value MaxAge. LSAs
having age MaxAge are not used in the routing table calculation. As
a router ages its link state database, an LSA's LS age may reach
MaxAge.[21] At this time, the router must attempt to flush the LSA
from the routing domain. This is done simply by reflooding the
MaxAge LSA just as if it was a newly originated LSA (see Section
13.3).

When creating a Database summary list for a newly forming adjacency,
any MaxAge LSAs present in the link state database are added to the
neighbor's Link state retransmission list instead of the neighbor's
Database summary list. See Section 10.3 for more details.

A MaxAge LSA must be removed immediately from the router's link
state database as soon as both a) it is no longer contained on any
neighbor Link state retransmission lists and b) none of the router's
neighbors are in states Exchange or Loading.

When, in the process of aging the link state database, an LSA's LS
age hits a multiple of CheckAge, its LS checksum should be verified.
If the LS checksum is incorrect, a program or memory error has been
detected, and at the very least the router itself should be
restarted.

14.1. Premature aging of LSAs

An LSA can be flushed from the routing domain by setting its LS
age to MaxAge, while leaving its LS sequence number alone, and
then reflooding the LSA. This procedure follows the same course
as flushing an LSA whose LS age has naturally reached the value
MaxAge (see Section 14). In particular, the MaxAge LSA is
removed from the router's link state database as soon as a) it
is no longer contained on any neighbor Link state retransmission
lists and b) none of the router's neighbors are in states
Exchange or Loading. We call the setting of an LSA's LS age to
MaxAge "premature aging".

Premature aging is used when it is time for a self-originated
LSA's sequence number field to wrap. At this point, the current
LSA instance (having LS sequence number MaxSequenceNumber) must
be prematurely aged and flushed from the routing domain before a
new instance with sequence number equal to InitialSequenceNumber
can be originated. See Section 12.1.6 for more information.

Premature aging can also be used when, for example, one of the
router's previously advertised external routes is no longer
reachable. In this circumstance, the router can flush its AS-
external-LSA from the routing domain via premature aging. This
procedure is preferable to the alternative, which is to
originate a new LSA for the destination specifying a metric of
LSInfinity. Premature aging is also be used when unexpectedly
receiving self-originated LSAs during the flooding procedure
(see Section 13.4).

A router may only prematurely age its own self-originated LSAs.
The router may not prematurely age LSAs that have been
originated by other routers. An LSA is considered self-
originated when either 1) the LSA's Advertising Router is equal
to the router's own Router ID or 2) the LSA is a network-LSA and
its Link State ID is equal to one of the router's own IP
interface addresses.

15. Virtual Links

The single backbone area (Area ID = 0.0.0.0) cannot be disconnected,
or some areas of the Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links can
be configured through non-backbone areas. Virtual links serve to
connect physically separate components of the backbone. The two
endpoints of a virtual link are area border routers. The virtual
link must be configured in both routers. The configuration
information in each router consists of the other virtual endpoint
(the other area border router), and the non-backbone area the two
routers have in common (called the Transit area). Virtual links
cannot be configured through stub areas (see Section 3.6).

The virtual link is treated as if it were an unnumbered point-to-
point network belonging to the backbone and joining the two area
border routers. An attempt is made to establish an adjacency over
the virtual link. When this adjacency is established, the virtual
link will be included in backbone router-LSAs, and OSPF packets
pertaining to the backbone area will flow over the adjacency. Such
an adjacency has been referred to in this document as a "virtual
adjacency".

In each endpoint router, the cost and viability of the virtual link
is discovered by examining the routing table entry for the other
endpoint router. (The entry's associated area must be the
configured Transit area). This is called the virtual link's
corresponding routing table entry. The InterfaceUp event occurs for
a virtual link when its corresponding routing table entry becomes
reachable. Conversely, the InterfaceDown event occurs when its
routing table entry becomes unreachable. In other words, the
virtual link's viability is determined by the existence of an
intra-area path, through the Transit area, between the two
endpoints. Note that a virtual link whose underlying path has cost
greater than hexadecimal 0xffff (the maximum size of an interface
cost in a router-LSA) should be considered inoperational (i.e.,
treated the same as if the path did not exist).

The other details concerning virtual links are as follows:

o AS-external-LSAs are NEVER flooded over virtual adjacencies.
This would be duplication of effort, since the same AS-

external-LSAs are already flooded throughout the virtual link's
Transit area. For this same reason, AS-external-LSAs are not
summarized over virtual adjacencies during the Database Exchange
process.

o The cost of a virtual link is NOT configured. It is defined to
be the cost of the intra-area path between the two defining area
border routers. This cost appears in the virtual link's
corresponding routing table entry. When the cost of a virtual
link changes, a new router-LSA should be originated for the
backbone area.

o Just as the virtual link's cost and viability are determined by
the routing table build process (through construction of the
routing table entry for the other endpoint), so are the IP
interface address for the virtual interface and the virtual
neighbor's IP address. These are used when sending OSPF
protocol packets over the virtual link. Note that when one (or
both) of the virtual link endpoints connect to the Transit area
via an unnumbered point-to-point link, it may be impossible to
calculate either the virtual interface's IP address and/or the
virtual neighbor's IP address, thereby causing the virtual link
to fail.

o In each endpoint's router-LSA for the backbone, the virtual link
is represented as a Type 4 link whose Link ID is set to the
virtual neighbor's OSPF Router ID and whose Link Data is set to
the virtual interface's IP address. See Section 12.4.1 for more
information.

o A non-backbone area can carry transit data traffic (i.e., is
considered a "transit area") if and only if it serves as the
Transit area for one or more fully adjacent virtual links (see
TransitCapability in Sections 6 and 16.1). Such an area requires
special treatment when summarizing backbone networks into it
(see Section 12.4.3), and during the routing calculation (see
Section 16.3).

o The time between link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the
expected round-trip delay between the two routers. This may be

hard to estimate for a virtual link; it is better to err on the
side of making it too large.

16. Calculation of the routing table

This section details the OSPF routing table calculation. Using its
attached areas' link state databases as input, a router runs the
following algorithm, building its routing table step by step. At
each step, the router must access individual pieces of the link
state databases (e.g., a router-LSA originated by a certain router).
This access is performed by the lookup function discussed in Section
12.2. The lookup process may return an LSA whose LS age is equal to
MaxAge. Such an LSA should not be used in the routing table
calculation, and is treated just as if the lookup process had
failed.

The OSPF routing table's organization is explained in Section 11.
Two examples of the routing table build process are presented in
Sections 11.2 and 11.3. This process can be broken into the
following steps:

(1) The present routing table is invalidated. The routing table is
built again from scratch. The old routing table is saved so
that changes in routing table entries can be identified.

(2) The intra-area routes are calculated by building the shortest-
path tree for each attached area. In particular, all routing
table entries whose Destination Type is "area border router" are
calculated in this step. This step is described in two parts.
At first the tree is constructed by only considering those links
between routers and transit networks. Then the stub networks
are incorporated into the tree. During the area's shortest-path
tree calculation, the area's TransitCapability is also
calculated for later use in Step 4.

(3) The inter-area routes are calculated, through examination of
summary-LSAs. If the router is attached to multiple areas
(i.e., it is an area border router), only backbone summary-LSAs
are examined.

(4) In area border routers connecting to one or more transit areas
(i.e, non-backbone areas whose TransitCapability is found to be
TRUE), the transit areas' summary-LSAs are examined to see
whether better paths exist using the transit areas than were
found in Steps 2-3 above.

(5) Routes to external destinations are calculated, through
examination of AS-external-LSAs. The locations of the AS
boundary routers (which originate the AS-external-LSAs) have
been determined in steps 2-4.

Steps 2-5 are explained in further detail below.

Changes made to routing table entries as a result of these
calculations can cause the OSPF protocol to take further actions.
For example, a change to an intra-area route will cause an area
border router to originate new summary-LSAs (see Section 12.4). See
Section 16.7 for a complete list of the OSPF protocol actions
resulting from routing table changes.

16.1. Calculating the shortest-path tree for an area

This calculation yields the set of intra-area routes associated
with an area (called hereafter Area A). A router calculates the
shortest-path tree using itself as the root.[22] The formation
of the shortest path tree is done here in two stages. In the
first stage, only links between routers and transit networks are
considered. Using the Dijkstra algorithm, a tree is formed from
this subset of the link state database. In the second stage,
leaves are added to the tree by considering the links to stub
networks.

The procedure will be explained using the graph terminology that
was introduced in Section 2. The area's link state database is
represented as a directed graph. The graph's vertices are
routers, transit networks and stub networks. The first stage of
the procedure concerns only the transit vertices (routers and
transit networks) and their connecting links. Throughout the
shortest path calculation, the following data is also associated
with each transit vertex:

Vertex (node) ID
A 32-bit number which together with the vertex type (router
or network) uniquely identifies the vertex. For router
vertices the Vertex ID is the router's OSPF Router ID. For
network vertices, it is the IP address of the network's
Designated Router.

An LSA
Each transit vertex has an associated LSA. For router
vertices, this is a router-LSA. For transit networks, this
is a network-LSA (which is actually originated by the
network's Designated Router). In any case, the LSA's Link
State ID is always equal to the above Vertex ID.

List of next hops
The list of next hops for the current set of shortest paths
from the root to this vertex. There can be multiple
shortest paths due to the equal-cost multipath capability.
Each next hop indicates the outgoing router interface to use
when forwarding traffic to the destination. On broadcast,
Point-to-MultiPoint and NBMA networks, the next hop also
includes the IP address of the next router (if any) in the
path towards the destination.

Distance from root
The link state cost of the current set of shortest paths
from the root to the vertex. The link state cost of a path
is calculated as the sum of the costs of the path's
constituent links (as advertised in router-LSAs and
network-LSAs). One path is said to be "shorter" than
another if it has a smaller link state cost.

The first stage of the procedure (i.e., the Dijkstra algorithm)
can now be summarized as follows. At each iteration of the
algorithm, there is a list of candidate vertices. Paths from
the root to these vertices have been found, but not necessarily
the shortest ones. However, the paths to the candidate vertex
that is closest to the root are guaranteed to be shortest; this
vertex is added to the shortest-path tree, removed from the
candidate list, and its adjacent vertices are examined for
possible addition to/modification of the candidate list. The

algorithm then iterates again. It terminates when the candidate
list becomes empty.

The following steps describe the algorithm in detail. Remember
that we are computing the shortest path tree for Area A. All
references to link state database lookup below are from Area A's
database.

(1) Initialize the algorithm's data structures. Clear the list
of candidate vertices. Initialize the shortest-path tree to
only the root (which is the router doing the calculation).
Set Area A's TransitCapability to FALSE.

(2) Call the vertex just added to the tree vertex V. Examine
the LSA associated with vertex V. This is a lookup in the
Area A's link state database based on the Vertex ID. If
this is a router-LSA, and bit V of the router-LSA (see
Section A.4.2) is set, set Area A's TransitCapability to
TRUE. In any case, each link described by the LSA gives the
cost to an adjacent vertex. For each described link, (say
it joins vertex V to vertex W):

(a) If this is a link to a stub network, examine the next
link in V's LSA. Links to stub networks will be
considered in the second stage of the shortest path
calculation.

(b) Otherwise, W is a transit vertex (router or transit
network). Look up the vertex W's LSA (router-LSA or
network-LSA) in Area A's link state database. If the
LSA does not exist, or its LS age is equal to MaxAge, or
it does not have a link back to vertex V, examine the
next link in V's LSA.[23]

(c) If vertex W is already on the shortest-path tree,
examine the next link in the LSA.

(d) Calculate the link state cost D of the resulting path
from the root to vertex W. D is equal to the sum of the
link state cost of the (already calculated) shortest
path to vertex V and the advertised cost of the link
between vertices V and W. If D is:

o Greater than the value that already appears for
vertex W on the candidate list, then examine the
next link.

o Equal to the value that appears for vertex W on the
candidate list, calculate the set of next hops that
result from using the advertised link. Input to
this calculation is the destination (W), and its
parent (V). This calculation is shown in Section
16.1.1. This set of hops should be added to the
next hop values that appear for W on the candidate
list.

o Less than the value that appears for vertex W on the
candidate list, or if W does not yet appear on the
candidate list, then set the entry for W on the
candidate list to indicate a distance of D from the
root. Also calculate the list of next hops that
result from using the advertised link, setting the
next hop values for W accordingly. The next hop
calculation is described in Section 16.1.1; it takes
as input the destination (W) and its parent (V).

(3) If at this step the candidate list is empty, the shortest-
path tree (of transit vertices) has been completely built
and this stage of the procedure terminates. Otherwise,
choose the vertex belonging to the candidate list that is
closest to the root, and add it to the shortest-path tree
(removing it from the candidate list in the process). Note
that when there is a choice of vertices closest to the root,
network vertices must be chosen before router vertices in
order to necessarily find all equal-cost paths. This is
consistent with the tie-breakers that were introduced in the
modified Dijkstra algorithm used by OSPF's Multicast routing
extensions (MOSPF).

(4) Possibly modify the routing table. For those routing table
entries modified, the associated area will be set to Area A,
the path type will be set to intra-area, and the cost will
be set to the newly discovered shortest path's calculated
distance.

If the newly added vertex is an area border router or AS
boundary router, a routing table entry is added whose
destination type is "router". The Options field found in
the associated router-LSA is copied into the routing table
entry's Optional capabilities field. Call the newly added
vertex Router X. If Router X is the endpoint of one of the
calculating router's virtual links, and the virtual link
uses Area A as Transit area: the virtual link is declared
up, the IP address of the virtual interface is set to the IP
address of the outgoing interface calculated above for
Router X, and the virtual neighbor's IP address is set to
Router X's interface address (contained in Router X's
router-LSA) that points back to the root of the shortest-
path tree; equivalently, this is the interface that points
back to Router X's parent vertex on the shortest-path tree
(similar to the calculation in Section 16.1.1).

If the newly added vertex is a transit network, the routing
table entry for the network is located. The entry's
Destination ID is the IP network number, which can be
obtained by masking the Vertex ID (Link State ID) with its
associated subnet mask (found in the body of the associated
network-LSA). If the routing table entry already exists
(i.e., there is already an intra-area route to the
destination installed in the routing table), multiple
vertices have mapped to the same IP network. For example,
this can occur when a new Designated Router is being
established. In this case, the current routing table entry
should be overwritten if and only if the newly found path is
just as short and the current routing table entry's Link
State Origin has a smaller Link State ID than the newly
added vertex' LSA.

If there is no routing table entry for the network (the
usual case), a routing table entry for the IP network should
be added. The routing table entry's Link State Origin
should be set to the newly added vertex' LSA.

(5) Iterate the algorithm by returning to Step 2.

The stub networks are added to the tree in the procedure's
second stage. In this stage, all router vertices are again
examined. Those that have been determined to be unreachable in
the above first phase are discarded. For each reachable router
vertex (call it V), the associated router-LSA is found in the
link state database. Each stub network link appearing in the
LSA is then examined, and the following steps are executed:

(1) Calculate the distance D of stub network from the root. D
is equal to the distance from the root to the router vertex
(calculated in stage 1), plus the stub network link's
advertised cost. Compare this distance to the current best
cost to the stub network. This is done by looking up the
stub network's current routing table entry. If the
calculated distance D is larger, go on to examine the next
stub network link in the LSA.

(2) If this step is reached, the stub network's routing table
entry must be updated. Calculate the set of next hops that
would result from using the stub network link. This
calculation is shown in Section 16.1.1; input to this
calculation is the destination (the stub network) and the
parent vertex (the router vertex). If the distance D is the
same as the current routing table cost, simply add this set
of next hops to the routing table entry's list of next hops.
In this case, the routing table already has a Link State
Origin. If this Link State Origin is a router-LSA whose
Link State ID is smaller than V's Router ID, reset the Link
State Origin to V's router-LSA.

Otherwise D is smaller than the routing table cost.
Overwrite the current routing table entry by setting the
routing table entry's cost to D, and by setting the entry's
list of next hops to the newly calculated set. Set the
routing table entry's Link State Origin to V's router-LSA.
Then go on to examine the next stub network link.

For all routing table entries added/modified in the second
stage, the associated area will be set to Area A and the path
type will be set to intra-area. When the list of reachable
router-LSAs is exhausted, the second stage is completed. At

this time, all intra-area routes associated with Area A have
been determined.

The specification does not require that the above two stage
method be used to calculate the shortest path tree. However, if
another algorithm is used, an identical tree must be produced.
For this reason, it is important to note that links between
transit vertices must be bidirectional in order to be included
in the above tree. It should also be mentioned that more
efficient algorithms exist for calculating the tree; for
example, the incremental SPF algorithm described in [Ref1].

16.1.1. The next hop calculation

This section explains how to calculate the current set of
next hops to use for a destination. Each next hop consists
of the outgoing interface to use in forwarding packets to
the destination together with the IP address of the next hop
router (if any). The next hop calculation is invoked each
time a shorter path to the destination is discovered. This
can happen in either stage of the shortest-path tree
calculation (see Section 16.1). In stage 1 of the
shortest-path tree calculation a shorter path is found as
the destination is added to the candidate list, or when the
destination's entry on the candidate list is modified (Step
2d of Stage 1). In stage 2 a shorter path is discovered
each time the destination's routing table entry is modified
(Step 2 of Stage 2).

The set of next hops to use for the destination may be
recalculated several times during the shortest-path tree
calculation, as shorter and shorter paths are discovered.
In the end, the destination's routing table entry will
always reflect the next hops resulting from the absolute
shortest path(s).

Input to the next hop calculation is a) the destination and
b) its parent in the current shortest path between the root
(the calculating router) and the destination. The parent is
always a transit vertex (i.e., always a router or a transit
network).

If there is at least one intervening router in the current
shortest path between the destination and the root, the
destination simply inherits the set of next hops from the
parent. Otherwise, there are two cases. In the first case,
the parent vertex is the root (the calculating router
itself). This means that the destination is either a
directly connected network or directly connected router.
The outgoing interface in this case is simply the OSPF
interface connecting to the destination network/router. If
the destination is a router which connects to the
calculating router via a Point-to-MultiPoint network, the
destination's next hop IP address(es) can be determined by
examining the destination's router-LSA: each link pointing
back to the calculating router and having a Link Data field
belonging to the Point-to-MultiPoint network provides an IP
address of the next hop router. If the destination is a
directly connected network, or a router which connects to
the calculating router via a point-to-point interface, no
next hop IP address is required. If the destination is a
router connected to the calculating router via a virtual
link, the setting of the next hop should be deferred until
the calculation in Section 16.3.

In the second case, the parent vertex is a network that
directly connects the calculating router to the destination
router. The list of next hops is then determined by
examining the destination's router-LSA. For each link in
the router-LSA that points back to the parent network, the
link's Link Data field provides the IP address of a next hop
router. The outgoing interface to use can then be derived
from the next hop IP address (or it can be inherited from
the parent network).

16.2. Calculating the inter-area routes

The inter-area routes are calculated by examining summary-LSAs.
If the router has active attachments to multiple areas, only
backbone summary-LSAs are examined. Routers attached to a
single area examine that area's summary-LSAs. In either case,
the summary-LSAs examined below are all part of a single area's
link state database (call it Area A).

Summary-LSAs are originated by the area border routers. Each
summary-LSA in Area A is considered in turn. Remember that the
destination described by a summary-LSA is either a network (Type
3 summary-LSAs) or an AS boundary router (Type 4 summary-LSAs).
For each summary-LSA:

(1) If the cost specified by the LSA is LSInfinity, or if the
LSA's LS age is equal to MaxAge, then examine the the next
LSA.

(2) If the LSA was originated by the calculating router itself,
examine the next LSA.

(3) If it is a Type 3 summary-LSA, and the collection of
destinations described by the summary-LSA equals one of the
router's configured area address ranges (see Section 3.5),
and the particular area address range is active, then the
summary-LSA should be ignored. "Active" means that there
are one or more reachable (by intra-area paths) networks
contained in the area range.

(4) Else, call the destination described by the LSA N (for Type
3 summary-LSAs, N's address is obtained by masking the LSA's
Link State ID with the network/subnet mask contained in the
body of the LSA), and the area border originating the LSA
BR. Look up the routing table entry for BR having Area A as
its associated area. If no such entry exists for router BR
(i.e., BR is unreachable in Area A), do nothing with this
LSA and consider the next in the list. Else, this LSA
describes an inter-area path to destination N, whose cost is
the distance to BR plus the cost specified in the LSA. Call
the cost of this inter-area path IAC.

(5) Next, look up the routing table entry for the destination N.
(If N is an AS boundary router, look up the "router" routing
table entry associated with Area A). If no entry exists for
N or if the entry's path type is "type 1 external" or "type
2 external", then install the inter-area path to N, with
associated area Area A, cost IAC, next hop equal to the list
of next hops to router BR, and Advertising router equal to
BR.

(6) Else, if the paths present in the table are intra-area
paths, do nothing with the LSA (intra-area paths are always
preferred).

(7) Else, the paths present in the routing table are also
inter-area paths. Install the new path through BR if it is
cheaper, overriding the paths in the routing table.
Otherwise, if the new path is the same cost, add it to the
list of paths that appear in the routing table entry.

16.3. Examining transit areas' summary-LSAs

This step is only performed by area border routers attached to
one or more non-backbone areas that are capable of carrying
transit traffic (i.e., "transit areas", or those areas whose
TransitCapability parameter has been set to TRUE in Step 2 of
the Dijkstra algorithm (see Section 16.1).

The purpose of the calculation below is to examine the transit
areas to see whether they provide any better (shorter) paths
than the paths previously calculated in Sections 16.1 and 16.2.
Any paths found that are better than or equal to previously
discovered paths are installed in the routing table.

The calculation also determines the actual next hop(s) for those
destinations whose next hop was calculated as a virtual link in
Sections 16.1 and 16.2. After completion of the calculation
below, any paths calculated in Sections 16.1 and 16.2 that still
have unresolved virtual next hops should be discarded.

The calculation proceeds as follows. All the transit areas'
summary-LSAs are examined in turn. Each such summary-LSA
describes a route through a transit area Area A to a Network N
(N's address is obtained by masking the LSA's Link State ID with
the network/subnet mask contained in the body of the LSA) or in
the case of a Type 4 summary-LSA, to an AS boundary router N.
Suppose also that the summary-LSA was originated by an area
border router BR.

(1) If the cost advertised by the summary-LSA is LSInfinity, or
if the LSA's LS age is equal to MaxAge, then examine the
next LSA.

(2) If the summary-LSA was originated by the calculating router
itself, examine the next LSA.

(3) Look up the routing table entry for N. (If N is an AS
boundary router, look up the "router" routing table entry
associated with the backbone area). If it does not exist, or
if the route type is other than intra-area or inter-area, or
if the area associated with the routing table entry is not
the backbone area, then examine the next LSA. In other
words, this calculation only updates backbone intra-area
routes found in Section 16.1 and inter-area routes found in
Section 16.2.

(4) Look up the routing table entry for the advertising router
BR associated with the Area A. If it is unreachable, examine
the next LSA. Otherwise, the cost to destination N is the
sum of the cost in BR's Area A routing table entry and the
cost advertised in the LSA. Call this cost IAC.

(5) If this cost is less than the cost occurring in N's routing
table entry, overwrite N's list of next hops with those used
for BR, and set N's routing table cost to IAC. Else, if IAC
is the same as N's current cost, add BR's list of next hops
to N's list of next hops. In any case, the area associated
with N's routing table entry must remain the backbone area,
and the path type (either intra-area or inter-area) must
also remain the same.

It is important to note that the above calculation never makes
unreachable destinations reachable, but instead just potentially
finds better paths to already reachable destinations. The
calculation installs any better cost found into the routing
table entry, from which it may be readvertised in summary-LSAs
to other areas.

As an example of the calculation, consider the Autonomous System
pictured in Figure 17. There is a single non-backbone area
(Area 1) that physically divides the backbone into two separate
pieces. To maintain connectivity of the backbone, a virtual link
has been configured between routers RT1 and RT4. On the right
side of the figure, Network N1 belongs to the backbone. The
dotted lines indicate that there is a much shorter intra-area

........................
. Area 1 (transit) . +
. .
. +---+1 1+---+100
. RT2----------RT4=========
. 1/+---+********* +---+
. /******* .
. 1/*Virtual .
1+---+/* Link . Network
=======RT1* . N1
+---+ .
. .
. .
. 1+---+1 1+---+20
. RT3----------RT5=========
. +---+ +---+
. .
........................ +

Figure 17: Routing through transit areas

backbone path between router RT5 and Network N1 (cost 20) than
there is between Router RT4 and Network N1 (cost 100). Both
Router RT4 and Router RT5 will inject summary-LSAs for Network
N1 into Area 1.

After the shortest-path tree has been calculated for the
backbone in Section 16.1, Router RT1 (left end of the virtual
link) will have calculated a path through Router RT4 for all
data traffic destined for Network N1. However, since Router RT5
is so much closer to Network N1, all routers internal to Area 1
(e.g., Routers RT2 and RT3) will forward their Network N1
traffic towards Router RT5, instead of RT4. And indeed, after
examining Area 1's summary-LSAs by the above calculation, Router
RT1 will also forward Network N1 traffic towards RT5. Note that
in this example the virtual link enables transit data traffic to
be forwarded through Area 1, but the actual path the transit
data traffic takes does not follow the virtual link. In other
words, virtual links allow transit traffic to be forwarded
through an area, but do not dictate the precise path that the
traffic will take.

16.4. Calculating AS external routes

AS external routes are calculated by examining AS-external-LSAs.
Each of the AS-external-LSAs is considered in turn. Most AS-
external-LSAs describe routes to specific IP destinations. An
AS-external-LSA can also describe a default route for the
Autonomous System (Destination ID = DefaultDestination,
network/subnet mask = 0x00000000). For each AS-external-LSA:

(1) If the cost specified by the LSA is LSInfinity, or if the
LSA's LS age is equal to MaxAge, then examine the next LSA.

(2) If the LSA was originated by the calculating router itself,
examine the next LSA.

(3) Call the destination described by the LSA N. N's address is
obtained by masking the LSA's Link State ID with the
network/subnet mask contained in the body of the LSA. Look
up the routing table entries (potentially one per attached
area) for the AS boundary router (ASBR) that originated the
LSA. If no entries exist for router ASBR (i.e., ASBR is
unreachable), do nothing with this LSA and consider the next
in the list.

Else, this LSA describes an AS external path to destination
N. Examine the forwarding address specified in the AS-
external-LSA. This indicates the IP address to which
packets for the destination should be forwarded.

If the forwarding address is set to 0.0.0.0, packets should
be sent to the ASBR itself. Among the multiple routing table
entries for the ASBR, select the preferred entry as follows.
If RFC1583Compatibility is set to "disabled", prune the set
of routing table entries for the ASBR as described in
Section 16.4.1. In any case, among the remaining routing
table entries, select the routing table entry with the least
cost; when there are multiple least cost routing table
entries the entry whose associated area has the largest OSPF
Area ID (when considered as an unsigned 32-bit integer) is
chosen.

If the forwarding address is non-zero, look up the
forwarding address in the routing table.[24] The matching
routing table entry must specify an intra-area or inter-area
path; if no such path exists, do nothing with the LSA and
consider the next in the list.

(4) Let X be the cost specified by the preferred routing table
entry for the ASBR/forwarding address, and Y the cost
specified in the LSA. X is in terms of the link state
metric, and Y is a type 1 or 2 external metric.

(5) Look up the routing table entry for the destination N. If
no entry exists for N, install the AS external path to N,
with next hop equal to the list of next hops to the
forwarding address, and advertising router equal to ASBR.
If the external metric type is 1, then the path-type is set
to type 1 external and the cost is equal to X+Y. If the
external metric type is 2, the path-type is set to type 2
external, the link state component of the route's cost is X,
and the type 2 cost is Y.

(6) Compare the AS external path described by the LSA with the
existing paths in N's routing table entry, as follows. If
the new path is preferred, it replaces the present paths in
N's routing table entry. If the new path is of equal
preference, it is added to N's routing table entry's list of
paths.

(a) Intra-area and inter-area paths are always preferred
over AS external paths.

(b) Type 1 external paths are always preferred over type 2
external paths. When all paths are type 2 external
paths, the paths with the smallest advertised type 2
metric are always preferred.

(c) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
and RFC1583Compatibility is set to "disabled", select
the preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section
16.4.1.

(d) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
select the preferred path based on a least cost
comparison. Type 1 external paths are compared by
looking at the sum of the distance to the forwarding
address and the advertised type 1 metric (X+Y). Type 2
external paths advertising equal type 2 metrics are
compared by looking at the distance to the forwarding
addresses.

16.4.1. External path preferences

When multiple intra-AS paths are available to
ASBRs/forwarding addresses, the following rules indicate
which paths are preferred. These rules apply when the same
ASBR is reachable through multiple areas, or when trying to
decide which of several AS-external-LSAs should be
preferred. In the former case the paths all terminate at the
same ASBR, while in the latter the paths terminate at
separate ASBRs/forwarding addresses. In either case, each
path is represented by a separate routing table entry as
defined in Section 11.

This section only applies when RFC1583Compatibility is set
to "disabled".

The path preference rules, stated from highest to lowest
preference, are as follows. Note that as a result of these
rules, there may still be multiple paths of the highest
preference. In this case, the path to use must be determined
based on cost, as described in Section 16.4.

o Intra-area paths using non-backbone areas are always the
most preferred.

o The other paths, intra-area backbone paths and inter-
area paths, are of equal preference.

16.5. Incremental updates -- summary-LSAs

When a new summary-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination

described by the summary-LSA N (N's address is obtained by
masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA), and let Area A be the area to
which the LSA belongs. There are then two separate cases:

Case 1: Area A is the backbone and/or the router is not an area
border router.
In this case, the following calculations must be performed.
First, if there is presently an inter-area route to the
destination N, N's routing table entry is invalidated,
saving the entry's values for later comparisons. Then the
calculation in Section 16.2 is run again for the single
destination N. In this calculation, all of Area A's
summary-LSAs that describe a route to N are examined. In
addition, if the router is an area border router attached to
one or more transit areas, the calculation in Section 16.3
must be run again for the single destination. If the
results of these calculations have changed the cost/path to
an AS boundary router (as would be the case for a Type 4
summary-LSA) or to any forwarding addresses, all AS-
external-LSAs will have to be reexamined by rerunning the
calculation in Section 16.4. Otherwise, if N is now newly
unreachable, the calculation in Section 16.4 must be rerun
for the single destination N, in case an alternate external
route to N exists.

Case 2: Area A is a transit area and the router is an area
border router.
In this case, the following calculations must be performed.
First, if N's routing table entry presently contains one or
more inter-area paths that utilize the transit area Area A,
these paths should be removed. If this removes all paths
from the routing table entry, the entry should be
invalidated. The entry's old values should be saved for
later comparisons. Next the calculation in Section 16.3 must
be run again for the single destination N. If the results of
this calculation have caused the cost to N to increase, the
complete routing table calculation must be rerun starting
with the Dijkstra algorithm specified in Section 16.1.
Otherwise, if the cost/path to an AS boundary router (as
would be the case for a Type 4 summary-LSA) or to any
forwarding addresses has changed, all AS-external-LSAs will

have to be reexamined by rerunning the calculation in
Section 16.4. Otherwise, if N is now newly unreachable, the
calculation in Section 16.4 must be rerun for the single
destination N, in case an alternate external route to N
exists.

16.6. Incremental updates -- AS-external-LSAs

When a new AS-external-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination
described by the AS-external-LSA N. N's address is obtained by
masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA. If there is already an intra-
area or inter-area route to the destination, no recalculation is
necessary (internal routes take precedence).

Otherwise, the procedure in Section 16.4 will have to be
performed, but only for those AS-external-LSAs whose destination
is N. Before this procedure is performed, the present routing
table entry for N should be invalidated.

16.7. Events generated as a result of routing table changes

Changes to routing table entries sometimes cause the OSPF area
border routers to take additional actions. These routers need
to act on the following routing table changes:

o The cost or path type of a routing table entry has changed.
If the destination described by this entry is a Network or
AS boundary router, and this is not simply a change of AS
external routes, new summary-LSAs may have to be generated
(potentially one for each attached area, including the
backbone). See Section 12.4.3 for more information. If a
previously advertised entry has been deleted, or is no
longer advertisable to a particular area, the LSA must be
flushed from the routing domain by setting its LS age to
MaxAge and reflooding (see Section 14.1).

o A routing table entry associated with a configured virtual
link has changed. The destination of such a routing table
entry is an area border router. The change indicates a
modification to the virtual link's cost or viability.

If the entry indicates that the area border router is newly
reachable, the corresponding virtual link is now
operational. An InterfaceUp event should be generated for
the virtual link, which will cause a virtual adjacency to
begin to form (see Section 10.3). At this time the virtual
link's IP interface address and the virtual neighbor's
Neighbor IP address are also calculated.

If the entry indicates that the area border router is no
longer reachable, the virtual link and its associated
adjacency should be destroyed. This means an InterfaceDown
event should be generated for the associated virtual link.

If the cost of the entry has changed, and there is a fully
established virtual adjacency, a new router-LSA for the
backbone must be originated. This in turn may cause further
routing table changes.

16.8. Equal-cost multipath

The OSPF protocol maintains multiple equal-cost routes to all
destinations. This can be seen in the steps used above to
calculate the routing table, and in the definition of the
routing table structure.

Each one of the multiple routes will be of the same type
(intra-area, inter-area, type 1 external or type 2 external),
cost, and will have the same associated area. However, each
route may specify a separate next hop and Advertising router.

There is no requirement that a router running OSPF keep track of
all possible equal-cost routes to a destination. An
implementation may choose to keep only a fixed number of routes
to any given destination. This does not affect any of the
algorithms presented in this specification.

Footnotes

[1]The graph's vertices represent either routers, transit networks,
or stub networks. Since routers may belong to multiple areas, it is
not possible to color the graph's vertices.

[2]It is possible for all of a router's interfaces to be unnumbered
point-to-point links. In this case, an IP address must be assigned
to the router. This address will then be advertised in the router's
router-LSA as a host route.

[3]Note that in these cases both interfaces, the non-virtual and the
virtual, would have the same IP address.

[4]Note that no host route is generated for, and no IP packets can
be addressed to, interfaces to unnumbered point-to-point networks.
This is regardless of such an interface's state.

[5]It is instructive to see what happens when the Designated Router
for the network crashes. Call the Designated Router for the network
RT1, and the Backup Designated Router RT2. If Router RT1 crashes
(or maybe its interface to the network dies), the other routers on
the network will detect RT1's absence within RouterDeadInterval
seconds. All routers may not detect this at precisely the same
time; the routers that detect RT1's absence before RT2 does will,
for a time, select RT2 to be both Designated Router and Backup
Designated Router. When RT2 detects that RT1 is gone it will move
itself to Designated Router. At this time, the remaining router
having highest Router Priority will be selected as Backup Designated
Router.

[6]On point-to-point networks, the lower level protocols indicate
whether the neighbor is up and running. Likewise, existence of the
neighbor on virtual links is indicated by the routing table
calculation. However, in both these cases, the Hello Protocol is
still used. This ensures that communication between the neighbors
is bidirectional, and that each of the neighbors has a functioning
routing protocol layer.

[7]When the identity of the Designated Router is changing, it may be
quite common for a neighbor in this state to send the router a

Database Description packet; this means that there is some momentary
disagreement on the Designated Router's identity.

[8]Note that it is possible for a router to resynchronize any of its
fully established adjacencies by setting the adjacency's state back
to ExStart. This will cause the other end of the adjacency to
process a SeqNumberMismatch event, and therefore to also go back to
ExStart state.

[9]The address space of IP networks and the address space of OSPF
Router IDs may overlap. That is, a network may have an IP address
which is identical (when considered as a 32-bit number) to some
router's Router ID.

[10]"Discard" entries are necessary to ensure that route
summarization at area boundaries will not cause packet looping.

[11]It is assumed that, for two different address ranges matching
the destination, one range is more specific than the other. Non-
contiguous subnet masks can be configured to violate this
assumption. Such subnet mask configurations cannot be handled by the
OSPF protocol.

[12]MaxAgeDiff is an architectural constant. It indicates the
maximum dispersion of ages, in seconds, that can occur for a single
LSA instance as it is flooded throughout the routing domain. If two
LSAs differ by more than this, they are assumed to be different
instances of the same LSA. This can occur when a router restarts
and loses track of the LSA's previous LS sequence number. See
Section 13.4 for more details.

[13]When two LSAs have different LS checksums, they are assumed to
be separate instances. This can occur when a router restarts, and
loses track of the LSA's previous LS sequence number. In the case
where the two LSAs have the same LS sequence number, it is not
possible to determine which LSA is actually newer. However, if the
wrong LSA is accepted as newer, the originating router will simply
originate another instance. See Section 13.4 for further details.

[14]There is one instance where a lookup must be done based on
partial information. This is during the routing table calculation,
when a network-LSA must be found based solely on its Link State ID.

The lookup in this case is still well defined, since no two
network-LSAs can have the same Link State ID.

[15]This is the way RFC1583 specified point-to-point
representation. It has three advantages: a) it does not require
allocating a subnet to the point-to-point link, b) it tends to bias
the routing so that packets destined for the point-to-point
interface will actually be received over the interface (which is
useful for diagnostic purposes) and c) it allows network
bootstrapping of a neighbor, without requiring that the bootstrap
program contain an OSPF implementation.

[16]This is the more traditional point-to-point representation used
by protocols such as RIP.

[17]This clause covers the case: Inter-area routes are not
summarized to the backbone. This is because inter-area routes are
always associated with the backbone area.

[18]This clause is only invoked when a non-backbone Area A supports
transit data traffic (i.e., has TransitCapability set to TRUE). For
example, in the area configuration of Figure 6, Area 2 can support
transit traffic due to the configured virtual link between Routers
RT10 and RT11. As a result, Router RT11 need only originate a single
summary-LSA into Area 2 (having the collapsed destination N9-
N11,H1), since all of Router RT11's other eligible routes have next
hops belonging to Area 2 itself (and as such only need be advertised
by other area border routers; in this case, Routers RT10 and RT7).

[19]By keeping more information in the routing table, it is possible
for an implementation to recalculate the shortest path tree for only
a single area. In fact, there are incremental algorithms that allow
an implementation to recalculate only a portion of a single area's
shortest path tree [Ref1]. However, these algorithms are beyond the
scope of this specification.

[20]This is how the Link state request list is emptied, which
eventually causes the neighbor state to transition to Full. See
Section 10.9 for more details.

[21]It should be a relatively rare occurrence for an LSA's LS age to
reach MaxAge in this fashion. Usually, the LSA will be replaced by

a more recent instance before it ages out.

[22]Strictly speaking, because of equal-cost multipath, the
algorithm does not create a tree. We continue to use the "tree"
terminology because that is what occurs most often in the existing
literature.

[23]Note that the presence of any link back to V is sufficient; it
need not be the matching half of the link under consideration from V
to W. This is enough to ensure that, before data traffic flows
between a pair of neighboring routers, their link state databases
will be synchronized.

[24]When the forwarding address is non-zero, it should point to a
router belonging to another Autonomous System. See Section 12.4.4
for more details.

References

[Ref1] McQuillan, J., I. Richer and E. Rosen, "ARPANET Routing
Algorithm Improvements", BBN Technical Report 3803, April
1978.

[Ref2] Digital Equipment Corporation, "Information processing
systems -- Data communications -- Intermediate System to
Intermediate System Intra-Domain Routing Protocol", October
1987.

[Ref3] McQuillan, J., et.al., "The New Routing Algorithm for the
ARPANET", IEEE Transactions on Communications, May 1980.

[Ref4] Perlman, R., "Fault-Tolerant Broadcast of Routing
Information", Computer Networks, December 1983.

[Ref5] Postel, J., "Internet Protocol", STD 5, RFC791, September
1981.

[Ref6] McKenzie, A., "ISO Transport Protocol specification ISO DP
8073", RFC905, April 1984.

[Ref7] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC1112, May 1988.

[Ref8] McCloghrie, K., and M. Rose, "Management Information Base
for network management of TCP/IP-based internets: MIB-II",
STD 17, RFC1213, March 1991.

[Ref9] Moy, J., "OSPF Version 2", RFC1583, March 1994.

[Ref10] Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
Inter-Domain Routing (CIDR): an Address Assignment and
Aggregation Strategy", RFC1519, September 1993.

[Ref11] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.

[Ref12] Almquist, P., "Type of Service in the Internet Protocol
Suite", RFC1349, July 1992.

[Ref13] Leiner, B., et.al., "The DARPA Internet Protocol Suite", DDN
Protocol Handbook, April 1985.

[Ref14] Bradley, T., and C. Brown, "Inverse Address Resolution
Protocol", RFC1293, January 1992.

[Ref15] deSouza, O., and M. Rodrigues, "Guidelines for Running OSPF
Over Frame Relay Networks", RFC1586, March 1994.

[Ref16] Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", ACM Computer Communications Review, Volume 19,
Number 2, pp. 32-38, April 1989.

[Ref17] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321,
April 1992.

[Ref18] Moy, J., "Multicast Extensions to OSPF", RFC1584, March
1994.

[Ref19] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC1587,
March 1994.

[Ref20] Ferguson, D., "The OSPF External Attributes LSA", work in
progress.

[Ref21] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
1793, April 1995.

[Ref22] Mogul, J., and S. Deering, "Path MTU Discovery", RFC1191,
November 1990.

[Ref23] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
4)", RFC1771, March 1995.

[Ref24] Hinden, R., "Internet Routing Protocol Standardization
Criteria", BBN, October 1991.

[Ref25] Moy, J., "OSPF Version 2", RFC2178, July 1997.

[Ref26] Rosen, E., "Vulnerabilities of Network Control Protocols: An
Example", Computer Communication Review, July 1981.

A. OSPF data formats

This appendix describes the format of OSPF protocol packets and OSPF
LSAs. The OSPF protocol runs directly over the IP network layer.
Before any data formats are described, the details of the OSPF
encapsulation are explained.

Next the OSPF Options field is described. This field describes
various capabilities that may or may not be supported by pieces of
the OSPF routing domain. The OSPF Options field is contained in OSPF
Hello packets, Database Description packets and in OSPF LSAs.

OSPF packet formats are detailed in Section A.3. A description of
OSPF LSAs appears in Section A.4.

A.1 Encapsulation of OSPF packets

OSPF runs directly over the Internet Protocol's network layer. OSPF
packets are therefore encapsulated solely by IP and local data-link
headers.

OSPF does not define a way to fragment its protocol packets, and
depends on IP fragmentation when transmitting packets larger than
the network MTU. If necessary, the length of OSPF packets can be up
to 65,535 bytes (including the IP header). The OSPF packet types
that are likely to be large (Database Description Packets, Link
State Request, Link State Update, and Link State Acknowledgment
packets) can usually be split into several separate protocol
packets, without loss of functionality. This is recommended; IP
fragmentation should be avoided whenever possible. Using this
reasoning, an attempt should be made to limit the sizes of OSPF
packets sent over virtual links to 576 bytes unless Path MTU
Discovery is being performed (see [Ref22]).

The other important features of OSPF's IP encapsulation are:

o Use of IP multicast. Some OSPF messages are multicast, when
sent over broadcast networks. Two distinct IP multicast
addresses are used. Packets sent to these multicast addresses
should never be forwarded; they are meant to travel a single hop
only. To ensure that these packets will not travel multiple
hops, their IP TTL must be set to 1.

AllSPFRouters
This multicast address has been assigned the value
224.0.0.5. All routers running OSPF should be prepared to
receive packets sent to this address. Hello packets are
always sent to this destination. Also, certain OSPF
protocol packets are sent to this address during the
flooding procedure.

AllDRouters
This multicast address has been assigned the value
224.0.0.6. Both the Designated Router and Backup Designated
Router must be prepared to receive packets destined to this
address. Certain OSPF protocol packets are sent to this
address during the flooding procedure.

o OSPF is IP protocol number 89. This number has been registered
with the Network Information Center. IP protocol number
assignments are documented in [Ref11].

o All OSPF routing protocol packets are sent using the normal
service TOS value of binary 0000 defined in [Ref12].

o Routing protocol packets are sent with IP precedence set to
Internetwork Control. OSPF protocol packets should be given
precedence over regular IP data traffic, in both sending and
receiving. Setting the IP precedence field in the IP header to
Internetwork Control [Ref5] may help implement this objective.

A.2 The Options field

The OSPF Options field is present in OSPF Hello packets, Database
Description packets and all LSAs. The Options field enables OSPF
routers to support (or not support) optional capabilities, and to
communicate their capability level to other OSPF routers. Through
this mechanism routers of differing capabilities can be mixed within
an OSPF routing domain.

When used in Hello packets, the Options field allows a router to
reject a neighbor because of a capability mismatch. Alternatively,
when capabilities are exchanged in Database Description packets a
router can choose not to forward certain LSAs to a neighbor because
of its reduced functionality. Lastly, listing capabilities in LSAs
allows routers to forward traffic around reduced functionality
routers, by excluding them from parts of the routing table
calculation.

Five bits of the OSPF Options field have been assigned, although
only one (the E-bit) is described completely by this memo. Each bit
is described briefly below. Routers should reset (i.e. clear)
unrecognized bits in the Options field when sending Hello packets or
Database Description packets and when originating LSAs. Conversely,
routers encountering unrecognized Option bits in received Hello
Packets, Database Description packets or LSAs should ignore the
capability and process the packet/LSA normally.

+------------------------------------+
* * DC EA N/P MC E *
+------------------------------------+

The Options field

E-bit
This bit describes the way AS-external-LSAs are flooded, as
described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo.

MC-bit
This bit describes whether IP multicast datagrams are forwarded
according to the specifications in [Ref18].

N/P-bit
This bit describes the handling of Type-7 LSAs, as specified in
[Ref19].

EA-bit
This bit describes the router's willingness to receive and
forward External-Attributes-LSAs, as specified in [Ref20].

DC-bit
This bit describes the router's handling of demand circuits, as
specified in [Ref21].

A.3 OSPF Packet Formats

There are five distinct OSPF packet types. All OSPF packet types
begin with a standard 24 byte header. This header is described
first. Each packet type is then described in a succeeding section.
In these sections each packet's division into fields is displayed,
and then the field definitions are enumerated.

All OSPF packet types (other than the OSPF Hello packets) deal with
lists of LSAs. For example, Link State Update packets implement the
flooding of LSAs throughout the OSPF routing domain. Because of
this, OSPF protocol packets cannot be parsed unless the format of
LSAs is also understood. The format of LSAs is described in Section
A.4.

The receive processing of OSPF packets is detailed in Section 8.2.
The sending of OSPF packets is explained in Section 8.1.

A.3.1 The OSPF packet header

Every OSPF packet starts with a standard 24 byte header. This
header contains all the information necessary to determine whether
the packet should be accepted for further processing. This
determination is described in Section 8.2 of the specification.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version # Type Packet length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Area ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum AuType
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Version #
The OSPF version number. This specification documents version 2
of the protocol.

Type
The OSPF packet types are as follows. See Sections A.3.2 through
A.3.6 for details.

Type Description
________________________________
1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment

Packet length
The length of the OSPF protocol packet in bytes. This length
includes the standard OSPF header.

Router ID
The Router ID of the packet's source.

Area ID
A 32 bit number identifying the area that this packet belongs
to. All OSPF packets are associated with a single area. Most
travel a single hop only. Packets travelling over a virtual
link are labelled with the backbone Area ID of 0.0.0.0.

Checksum
The standard IP checksum of the entire contents of the packet,
starting with the OSPF packet header but excluding the 64-bit
authentication field. This checksum is calculated as the 16-bit
one's complement of the one's complement sum of all the 16-bit
words in the packet, excepting the authentication field. If the
packet's length is not an integral number of 16-bit words, the
packet is padded with a byte of zero before checksumming. The
checksum is considered to be part of the packet authentication
procedure; for some authentication types the checksum
calculation is omitted.

AuType
Identifies the authentication procedure to be used for the
packet. Authentication is discussed in Appendix D of the
specification. Consult Appendix D for a list of the currently
defined authentication types.

Authentication
A 64-bit field for use by the authentication scheme. See
Appendix D for details.

A.3.2 The Hello packet

Hello packets are OSPF packet type 1. These packets are sent
periodically on all interfaces (including virtual links) in order to
establish and maintain neighbor relationships. In addition, Hello
Packets are multicast on those physical networks having a multicast
or broadcast capability, enabling dynamic discovery of neighboring
routers.

All routers connected to a common network must agree on certain
parameters (Network mask, HelloInterval and RouterDeadInterval).
These parameters are included in Hello packets, so that differences
can inhibit the forming of neighbor relationships. A detailed
explanation of the receive processing for Hello packets is presented
in Section 10.5. The sending of Hello packets is covered in Section
9.5.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version # 1 Packet length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Area ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum AuType
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network Mask
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
HelloInterval Options Rtr Pri
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
RouterDeadInterval
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Designated Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Backup Designated Router

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Neighbor
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

Network mask
The network mask associated with this interface. For example,
if the interface is to a class B network whose third byte is
used for subnetting, the network mask is 0xffffff00.

Options
The optional capabilities supported by the router, as documented
in Section A.2.

HelloInterval
The number of seconds between this router's Hello packets.

Rtr Pri
This router's Router Priority. Used in (Backup) Designated
Router election. If set to 0, the router will be ineligible to
become (Backup) Designated Router.

RouterDeadInterval
The number of seconds before declaring a silent router down.

Designated Router
The identity of the Designated Router for this network, in the
view of the sending router. The Designated Router is identified
here by its IP interface address on the network. Set to 0.0.0.0
if there is no Designated Router.

Backup Designated Router
The identity of the Backup Designated Router for this network,
in the view of the sending router. The Backup Designated Router
is identified here by its IP interface address on the network.
Set to 0.0.0.0 if there is no Backup Designated Router.

Neighbor
The Router IDs of each router from whom valid Hello packets have
been seen recently on the network. Recently means in the last
RouterDeadInterval seconds.

A.3.3 The Database Description packet

Database Description packets are OSPF packet type 2. These packets
are exchanged when an adjacency is being initialized. They describe
the contents of the link-state database. Multiple packets may be
used to describe the database. For this purpose a poll-response
procedure is used. One of the routers is designated to be the
master, the other the slave. The master sends Database Description
packets (polls) which are acknowledged by Database Description
packets sent by the slave (responses). The responses are linked to
the polls via the packets' DD sequence numbers.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version # 2 Packet length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Area ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum AuType
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Interface MTU Options 00000IMMS
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
DD sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+- -+

+- An LSA Header -+

+- -+

+- -+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

The format of the Database Description packet is very similar to
both the Link State Request and Link State Acknowledgment packets.
The main part of all three is a list of items, each item describing
a piece of the link-state database. The sending of Database
Description Packets is documented in Section 10.8. The reception of
Database Description packets is documented in Section 10.6.

Interface MTU
The size in bytes of the largest IP datagram that can be sent
out the associated interface, without fragmentation. The MTUs
of common Internet link types can be found in Table 7-1 of
[Ref22]. Interface MTU should be set to 0 in Database
Description packets sent over virtual links.

Options
The optional capabilities supported by the router, as documented
in Section A.2.

I-bit
The Init bit. When set to 1, this packet is the first in the
sequence of Database Description Packets.

M-bit
The More bit. When set to 1, it indicates that more Database
Description Packets are to follow.

MS-bit
The Master/Slave bit. When set to 1, it indicates that the
router is the master during the Database Exchange process.
Otherwise, the router is the slave.

DD sequence number
Used to sequence the collection of Database Description Packets.
The initial value (indicated by the Init bit being set) should
be unique. The DD sequence number then increments until the
complete database description has been sent.

The rest of the packet consists of a (possibly partial) list of the
link-state database's pieces. Each LSA in the database is described
by its LSA header. The LSA header is documented in Section A.4.1.
It contains all the information required to uniquely identify both
the LSA and the LSA's current instance.

A.3.4 The Link State Request packet

Link State Request packets are OSPF packet type 3. After exchanging
Database Description packets with a neighboring router, a router may
find that parts of its link-state database are out-of-date. The
Link State Request packet is used to request the pieces of the
neighbor's database that are more up-to-date. Multiple Link State
Request packets may need to be used.

A router that sends a Link State Request packet has in mind the
precise instance of the database pieces it is requesting. Each
instance is defined by its LS sequence number, LS checksum, and LS
age, although these fields are not specified in the Link State
Request Packet itself. The router may receive even more recent
instances in response.

The sending of Link State Request packets is documented in Section
10.9. The reception of Link State Request packets is documented in
Section 10.7.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version # 3 Packet length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Area ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum AuType
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS type
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link State ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Advertising Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

Each LSA requested is specified by its LS type, Link State ID, and
Advertising Router. This uniquely identifies the LSA, but not its
instance. Link State Request packets are understood to be requests
for the most recent instance (whatever that might be).

A.3.5 The Link State Update packet

Link State Update packets are OSPF packet type 4. These packets
implement the flooding of LSAs. Each Link State Update packet
carries a collection of LSAs one hop further from their origin.
Several LSAs may be included in a single packet.

Link State Update packets are multicast on those physical networks
that support multicast/broadcast. In order to make the flooding
procedure reliable, flooded LSAs are acknowledged in Link State
Acknowledgment packets. If retransmission of certain LSAs is
necessary, the retransmitted LSAs are always sent directly to the
neighbor. For more information on the reliable flooding of LSAs,
consult Section 13.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version # 4 Packet length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Area ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum AuType
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
# LSAs
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+- +-+
LSAs
+- +-+
...

# LSAs
The number of LSAs included in this update.

The body of the Link State Update packet consists of a list of LSAs.
Each LSA begins with a common 20 byte header, described in Section
A.4.1. Detailed formats of the different types of LSAs are described
in Section A.4.

A.3.6 The Link State Acknowledgment packet

Link State Acknowledgment Packets are OSPF packet type 5. To make
the flooding of LSAs reliable, flooded LSAs are explicitly
acknowledged. This acknowledgment is accomplished through the
sending and receiving of Link State Acknowledgment packets.
Multiple LSAs can be acknowledged in a single Link State
Acknowledgment packet.

Depending on the state of the sending interface and the sender of
the corresponding Link State Update packet, a Link State
Acknowledgment packet is sent either to the multicast address
AllSPFRouters, to the multicast address AllDRouters, or as a
unicast. The sending of Link State Acknowledgement packets is
documented in Section 13.5. The reception of Link State
Acknowledgement packets is documented in Section 13.7.

The format of this packet is similar to that of the Data Description
packet. The body of both packets is simply a list of LSA headers.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version # 5 Packet length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Area ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Checksum AuType
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+- -+

+- An LSA Header -+

+- -+


+- -+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

Each acknowledged LSA is described by its LSA header. The LSA
header is documented in Section A.4.1. It contains all the
information required to uniquely identify both the LSA and the LSA's
current instance.

A.4 LSA formats

This memo defines five distinct types of LSAs. Each LSA begins with
a standard 20 byte LSA header. This header is explained in Section
A.4.1. Succeeding sections then diagram the separate LSA types.

Each LSA describes a piece of the OSPF routing domain. Every router
originates a router-LSA. In addition, whenever the router is
elected Designated Router, it originates a network-LSA. Other types
of LSAs may also be originated (see Section 12.4). All LSAs are
then flooded throughout the OSPF routing domain. The flooding
algorithm is reliable, ensuring that all routers have the same
collection of LSAs. (See Section 13 for more information concerning
the flooding algorithm). This collection of LSAs is called the
link-state database.

From the link state database, each router constructs a shortest path
tree with itself as root. This yields a routing table (see Section
11). For the details of the routing table build process, see
Section 16.

A.4.1 The LSA header

All LSAs begin with a common 20 byte header. This header contains
enough information to uniquely identify the LSA (LS type, Link State
ID, and Advertising Router). Multiple instances of the LSA may
exist in the routing domain at the same time. It is then necessary
to determine which instance is more recent. This is accomplished by
examining the LS age, LS sequence number and LS checksum fields that
are also contained in the LSA header.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS age Options LS type
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link State ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Advertising Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS checksum length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

LS age
The time in seconds since the LSA was originated.

Options
The optional capabilities supported by the described portion of
the routing domain. OSPF's optional capabilities are documented
in Section A.2.

LS type
The type of the LSA. Each LSA type has a separate advertisement
format. The LSA types defined in this memo are as follows (see
Section 12.1.3 for further explanation):

LS Type Description
___________________________________
1 Router-LSAs
2 Network-LSAs
3 Summary-LSAs (IP network)
4 Summary-LSAs (ASBR)
5 AS-external-LSAs

Link State ID
This field identifies the portion of the internet environment
that is being described by the LSA. The contents of this field
depend on the LSA's LS type. For example, in network-LSAs the
Link State ID is set to the IP interface address of the
network's Designated Router (from which the network's IP address
can be derived). The Link State ID is further discussed in
Section 12.1.4.

Advertising Router
The Router ID of the router that originated the LSA. For
example, in network-LSAs this field is equal to the Router ID of
the network's Designated Router.

LS sequence number
Detects old or duplicate LSAs. Successive instances of an LSA
are given successive LS sequence numbers. See Section 12.1.6
for more details.

LS checksum
The Fletcher checksum of the complete contents of the LSA,
including the LSA header but excluding the LS age field. See
Section 12.1.7 for more details.

length
The length in bytes of the LSA. This includes the 20 byte LSA
header.

A.4.2 Router-LSAs

Router-LSAs are the Type 1 LSAs. Each router in an area originates
a router-LSA. The LSA describes the state and cost of the router's
links (i.e., interfaces) to the area. All of the router's links to
the area must be described in a single router-LSA. For details
concerning the construction of router-LSAs, see Section 12.4.1.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS age Options 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link State ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Advertising Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS checksum length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 VEB 0 # links
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type # TOS metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TOS 0 TOS metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

In router-LSAs, the Link State ID field is set to the router's OSPF
Router ID. Router-LSAs are flooded throughout a single area only.

bit V
When set, the router is an endpoint of one or more fully
adjacent virtual links having the described area as Transit area
(V is for virtual link endpoint).

bit E
When set, the router is an AS boundary router (E is for
external).

bit B
When set, the router is an area border router (B is for border).

# links
The number of router links described in this LSA. This must be
the total collection of router links (i.e., interfaces) to the
area.

The following fields are used to describe each router link (i.e.,
interface). Each router link is typed (see the below Type field).
The Type field indicates the kind of link being described. It may
be a link to a transit network, to another router or to a stub
network. The values of all the other fields describing a router
link depend on the link's Type. For example, each link has an
associated 32-bit Link Data field. For links to stub networks this
field specifies the network's IP address mask. For other link types
the Link Data field specifies the router interface's IP address.

Type
A quick description of the router link. One of the following.
Note that host routes are classified as links to stub networks
with network mask of 0xffffffff.

Type Description
__________________________________________________
1 Point-to-point connection to another router
2 Connection to a transit network
3 Connection to a stub network
4 Virtual link

Link ID
Identifies the object that this router link connects to. Value
depends on the link's Type. When connecting to an object that
also originates an LSA (i.e., another router or a transit
network) the Link ID is equal to the neighboring LSA's Link
State ID. This provides the key for looking up the neighboring
LSA in the link state database during the routing table
calculation. See Section 12.2 for more details.

Type Link ID
______________________________________
1 Neighboring router's Router ID
2 IP address of Designated Router
3 IP network/subnet number
4 Neighboring router's Router ID

Link Data
Value again depends on the link's Type field. For connections to
stub networks, Link Data specifies the network's IP address
mask. For unnumbered point-to-point connections, it specifies
the interface's MIB-II [Ref8] ifIndex value. For the other link
types it specifies the router interface's IP address. This
latter piece of information is needed during the routing table
build process, when calculating the IP address of the next hop.
See Section 16.1.1 for more details.

# TOS
The number of different TOS metrics given for this link, not
counting the required link metric (referred to as the TOS 0
metric in [Ref9]). For example, if no additional TOS metrics
are given, this field is set to 0.

metric
The cost of using this router link.

Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). Within each link, and for each desired TOS,
TOS TOS-specific link information may be encoded as follows:

TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF LSAs is described in Section 12.3.

TOS metric
TOS-specific metric information.

A.4.3 Network-LSAs

Network-LSAs are the Type 2 LSAs. A network-LSA is originated for
each broadcast and NBMA network in the area which supports two or
more routers. The network-LSA is originated by the network's
Designated Router. The LSA describes all routers attached to the
network, including the Designated Router itself. The LSA's Link
State ID field lists the IP interface address of the Designated
Router.

The distance from the network to all attached routers is zero. This
is why metric fields need not be specified in the network-LSA. For
details concerning the construction of network-LSAs, see Section
12.4.2.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS age Options 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link State ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Advertising Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS checksum length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network Mask
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Attached Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

Network Mask
The IP address mask for the network. For example, a class A
network would have the mask 0xff000000.

Attached Router
The Router IDs of each of the routers attached to the network.
Actually, only those routers that are fully adjacent to the
Designated Router are listed. The Designated Router includes
itself in this list. The number of routers included can be
deduced from the LSA header's length field.

A.4.4 Summary-LSAs

Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated
by area border routers. Summary-LSAs describe inter-area
destinations. For details concerning the construction of summary-
LSAs, see Section 12.4.3.

Type 3 summary-LSAs are used when the destination is an IP network.
In this case the LSA's Link State ID field is an IP network number
(if necessary, the Link State ID can also have one or more of the
network's "host" bits set; see Appendix E for details). When the
destination is an AS boundary router, a Type 4 summary-LSA is used,
and the Link State ID field is the AS boundary router's OSPF Router
ID. (To see why it is necessary to advertise the location of each
ASBR, consult Section 16.4.) Other than the difference in the Link
State ID field, the format of Type 3 and 4 summary-LSAs is
identical.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS age Options 3 or 4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link State ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Advertising Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS checksum length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network Mask
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TOS TOS metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

For stub areas, Type 3 summary-LSAs can also be used to describe a
(per-area) default route. Default summary routes are used in stub
areas instead of flooding a complete set of external routes. When
describing a default summary route, the summary-LSA's Link State ID
is always set to DefaultDestination (0.0.0.0) and the Network Mask
is set to 0.0.0.0.

Network Mask
For Type 3 summary-LSAs, this indicates the destination
network's IP address mask. For example, when advertising the
location of a class A network the value 0xff000000 would be
used. This field is not meaningful and must be zero for Type 4
summary-LSAs.

metric
The cost of this route. Expressed in the same units as the
interface costs in the router-LSAs.

Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). For each desired TOS, TOS-specific
information is encoded as follows:

TOS IP Type of Service that this metric refers to. The encoding of
TOS in OSPF LSAs is described in Section 12.3.

TOS metric
TOS-specific metric information.

A.4.5 AS-external-LSAs

AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by
AS boundary routers, and describe destinations external to the AS.
For details concerning the construction of AS-external-LSAs, see
Section 12.4.3.

AS-external-LSAs usually describe a particular external destination.
For these LSAs the Link State ID field specifies an IP network
number (if necessary, the Link State ID can also have one or more of
the network's "host" bits set; see Appendix E for details). AS-
external-LSAs are also used to describe a default route. Default
routes are used when no specific route exists to the destination.
When describing a default route, the Link State ID is always set to
DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS age Options 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Link State ID
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Advertising Router
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
LS checksum length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Network Mask
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
E 0 metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Forwarding address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
External Route Tag
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
E TOS TOS metric
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Forwarding address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

External Route Tag
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...

Network Mask
The IP address mask for the advertised destination. For
example, when advertising a class A network the mask 0xff000000
would be used.

bit E
The type of external metric. If bit E is set, the metric
specified is a Type 2 external metric. This means the metric is
considered larger than any link state path. If bit E is zero,
the specified metric is a Type 1 external metric. This means
that it is expressed in the same units as the link state metric
(i.e., the same units as interface cost).

metric
The cost of this route. Interpretation depends on the external
type indication (bit E above).

Forwarding address
Data traffic for the advertised destination will be forwarded to
this address. If the Forwarding address is set to 0.0.0.0, data
traffic will be forwarded instead to the LSA's originator (i.e.,
the responsible AS boundary router).

External Route Tag
A 32-bit field attached to each external route. This is not
used by the OSPF protocol itself. It may be used to communicate
information between AS boundary routers; the precise nature of
such information is outside the scope of this specification.

Additional TOS-specific information may also be included, for
backward compatibility with previous versions of the OSPF
specification ([Ref9]). For each desired TOS, TOS-specific
information is encoded as follows:

TOS The Type of Service that the following fields concern. The
encoding of TOS in OSPF LSAs is described in Section 12.3.

bit E
For backward-compatibility with [Ref9].

TOS metric
TOS-specific metric information.

Forwarding address
For backward-compatibility with [Ref9].

External Route Tag
For backward-compatibility with [Ref9].

B. Architectural Constants

Several OSPF protocol parameters have fixed architectural values.
These parameters have been referred to in the text by names such as
LSRefreshTime. The same naming convention is used for the
configurable protocol parameters. They are defined in Appendix C.

The name of each architectural constant follows, together with its
value and a short description of its function.

LSRefreshTime
The maximum time between distinct originations of any particular
LSA. If the LS age field of one of the router's self-originated
LSAs reaches the value LSRefreshTime, a new instance of the LSA
is originated, even though the contents of the LSA (apart from
the LSA header) will be the same. The value of LSRefreshTime is
set to 30 minutes.

MinLSInterval
The minimum time between distinct originations of any particular
LSA. The value of MinLSInterval is set to 5 seconds.

MinLSArrival
For any particular LSA, the minimum time that must elapse
between reception of new LSA instances during flooding. LSA
instances received at higher frequencies are discarded. The
value of MinLSArrival is set to 1 second.

MaxAge
The maximum age that an LSA can attain. When an LSA's LS age
field reaches MaxAge, it is reflooded in an attempt to flush the
LSA from the routing domain (See Section 14). LSAs of age MaxAge
are not used in the routing table calculation. The value of
MaxAge is set to 1 hour.

CheckAge
When the age of an LSA in the link state database hits a
multiple of CheckAge, the LSA's checksum is verified. An
incorrect checksum at this time indicates a serious error. The
value of CheckAge is set to 5 minutes.

MaxAgeDiff
The maximum time dispersion that can occur, as an LSA is flooded
throughout the AS. Most of this time is accounted for by the
LSAs sitting on router output queues (and therefore not aging)
during the flooding process. The value of MaxAgeDiff is set to
15 minutes.

LSInfinity
The metric value indicating that the destination described by an
LSA is unreachable. Used in summary-LSAs and AS-external-LSAs as
an alternative to premature aging (see Section 14.1). It is
defined to be the 24-bit binary value of all ones: 0xffffff.

DefaultDestination
The Destination ID that indicates the default route. This route
is used when no other matching routing table entry can be found.
The default destination can only be advertised in AS-external-
LSAs and in stub areas' type 3 summary-LSAs. Its value is the
IP address 0.0.0.0. Its associated Network Mask is also always
0.0.0.0.

InitialSequenceNumber
The value used for LS Sequence Number when originating the first
instance of any LSA. Its value is the signed 32-bit integer
0x80000001.

MaxSequenceNumber
The maximum value that LS Sequence Number can attain. Its value
is the signed 32-bit integer 0x7fffffff.

C. Configurable Constants

The OSPF protocol has quite a few configurable parameters. These
parameters are listed below. They are grouped into general
functional categories (area parameters, interface parameters, etc.).
Sample values are given for some of the parameters.

Some parameter settings need to be consistent among groups of
routers. For example, all routers in an area must agree on that
area's parameters, and all routers attached to a network must agree
on that network's IP network number and mask.

Some parameters may be determined by router algorithms outside of
this specification (e.g., the address of a host connected to the
router via a SLIP line). From OSPF's point of view, these items are
still configurable.

C.1 Global parameters

In general, a separate copy of the OSPF protocol is run for each
area. Because of this, most configuration parameters are
defined on a per-area basis. The few global configuration
parameters are listed below.

Router ID
This is a 32-bit number that uniquely identifies the router
in the Autonomous System. One algorithm for Router ID
assignment is to choose the largest or smallest IP address
assigned to the router. If a router's OSPF Router ID is
changed, the router's OSPF software should be restarted
before the new Router ID takes effect. Before restarting in
order to change its Router ID, the router should flush its
self-originated LSAs from the routing domain (see Section
14.1), or they will persist for up to MaxAge minutes.

RFC1583Compatibility
Controls the preference rules used in Section 16.4 when
choosing among multiple AS-external-LSAs advertising the
same destination. When set to "enabled", the preference
rules remain those specified by RFC1583 ([Ref9]). When set
to "disabled", the preference rules are those stated in

Section 16.4.1, which prevent routing loops when AS-
external-LSAs for the same destination have been originated
from different areas. Set to "enabled" by default.

In order to minimize the chance of routing loops, all OSPF
routers in an OSPF routing domain should have
RFC1583Compatibility set identically. When there are routers
present that have not been updated with the functionality
specified in Section 16.4.1 of this memo, all routers should
have RFC1583Compatibility set to "enabled". Otherwise, all
routers should have RFC1583Compatibility set to "disabled",
preventing all routing loops.

C.2 Area parameters

All routers belonging to an area must agree on that area's
configuration. Disagreements between two routers will lead to
an inability for adjacencies to form between them, with a
resulting hindrance to the flow of routing protocol and data
traffic. The following items must be configured for an area:

Area ID
This is a 32-bit number that identifies the area. The Area
ID of 0.0.0.0 is reserved for the backbone. If the area
represents a subnetted network, the IP network number of the
subnetted network may be used for the Area ID.

List of address ranges
An OSPF area is defined as a list of address ranges. Each
address range consists of the following items:

[IP address, mask]
Describes the collection of IP addresses contained
in the address range. Networks and hosts are
assigned to an area depending on whether their
addresses fall into one of the area's defining
address ranges. Routers are viewed as belonging to
multiple areas, depending on their attached
networks' area membership.

Status Set to either Advertise or DoNotAdvertise. Routing
information is condensed at area boundaries.
External to the area, at most a single route is
advertised (via a summary-LSA) for each address
range. The route is advertised if and only if the
address range's Status is set to Advertise.
Unadvertised ranges allow the existence of certain
networks to be intentionally hidden from other
areas. Status is set to Advertise by default.

As an example, suppose an IP subnetted network is to be its
own OSPF area. The area would be configured as a single
address range, whose IP address is the address of the
subnetted network, and whose mask is the natural class A, B,
or C address mask. A single route would be advertised
external to the area, describing the entire subnetted
network.

ExternalRoutingCapability
Whether AS-external-LSAs will be flooded into/throughout the
area. If AS-external-LSAs are excluded from the area, the
area is called a "stub". Internal to stub areas, routing to
external destinations will be based solely on a default
summary route. The backbone cannot be configured as a stub
area. Also, virtual links cannot be configured through stub
areas. For more information, see Section 3.6.

StubDefaultCost
If the area has been configured as a stub area, and the
router itself is an area border router, then the
StubDefaultCost indicates the cost of the default summary-
LSA that the router should advertise into the area.

C.3 Router interface parameters

Some of the configurable router interface parameters (such as IP
interface address and subnet mask) actually imply properties of
the attached networks, and therefore must be consistent across
all the routers attached to that network. The parameters that
must be configured for a router interface are:

IP interface address
The IP protocol address for this interface. This uniquely
identifies the router over the entire internet. An IP
address is not required on point-to-point networks. Such a
point-to-point network is called "unnumbered".

IP interface mask
Also referred to as the subnet/network mask, this indicates
the portion of the IP interface address that identifies the
attached network. Masking the IP interface address with the
IP interface mask yields the IP network number of the
attached network. On point-to-point networks and virtual
links, the IP interface mask is not defined. On these
networks, the link itself is not assigned an IP network
number, and so the addresses of each side of the link are
assigned independently, if they are assigned at all.

Area ID
The OSPF area to which the attached network belongs.

Interface output cost
The cost of sending a packet on the interface, expressed in
the link state metric. This is advertised as the link cost
for this interface in the router's router-LSA. The interface
output cost must always be greater than 0.

RxmtInterval
The number of seconds between LSA retransmissions, for
adjacencies belonging to this interface. Also used when
retransmitting Database Description and Link State Request
Packets. This should be well over the expected round-trip
delay between any two routers on the attached network. The
setting of this value should be conservative or needless
retransmissions will result. Sample value for a local area
network: 5 seconds.

InfTransDelay
The estimated number of seconds it takes to transmit a Link
State Update Packet over this interface. LSAs contained in
the update packet must have their age incremented by this
amount before transmission. This value should take into
account the transmission and propagation delays of the

interface. It must be greater than 0. Sample value for a
local area network: 1 second.

Router Priority
An 8-bit unsigned integer. When two routers attached to a
network both attempt to become Designated Router, the one
with the highest Router Priority takes precedence. If there
is still a tie, the router with the highest Router ID takes
precedence. A router whose Router Priority is set to 0 is
ineligible to become Designated Router on the attached
network. Router Priority is only configured for interfaces
to broadcast and NBMA networks.

HelloInterval
The length of time, in seconds, between the Hello Packets
that the router sends on the interface. This value is
advertised in the router's Hello Packets. It must be the
same for all routers attached to a common network. The
smaller the HelloInterval, the faster topological changes
will be detected; however, more OSPF routing protocol
traffic will ensue. Sample value for a X.25 PDN network: 30
seconds. Sample value for a local area network: 10 seconds.

RouterDeadInterval
After ceasing to hear a router's Hello Packets, the number
of seconds before its neighbors declare the router down.
This is also advertised in the router's Hello Packets in
their RouterDeadInterval field. This should be some
multiple of the HelloInterval (say 4). This value again
must be the same for all routers attached to a common
network.

AuType
Identifies the authentication procedure to be used on the
attached network. This value must be the same for all
routers attached to the network. See Appendix D for a
discussion of the defined authentication types.

Authentication key
This configured data allows the authentication procedure to
verify OSPF protocol packets received over the interface.
For example, if the AuType indicates simple password, the

Authentication key would be a clear 64-bit password.
Authentication keys associated with the other OSPF
authentication types are discussed in Appendix D.

C.4 Virtual link parameters

Virtual links are used to restore/increase connectivity of the
backbone. Virtual links may be configured between any pair of
area border routers having interfaces to a common (non-backbone)
area. The virtual link appears as an unnumbered point-to-point
link in the graph for the backbone. The virtual link must be
configured in both of the area border routers.

A virtual link appears in router-LSAs (for the backbone) as if
it were a separate router interface to the backbone. As such,
it has all of the parameters associated with a router interface
(see Section C.3). Although a virtual link acts like an
unnumbered point-to-point link, it does have an associated IP
interface address. This address is used as the IP source in
OSPF protocol packets it sends along the virtual link, and is
set dynamically during the routing table build process.
Interface output cost is also set dynamically on virtual links
to be the cost of the intra-area path between the two routers.
The parameter RxmtInterval must be configured, and should be
well over the expected round-trip delay between the two routers.
This may be hard to estimate for a virtual link; it is better to
err on the side of making it too large. Router Priority is not
used on virtual links.

A virtual link is defined by the following two configurable
parameters: the Router ID of the virtual link's other endpoint,
and the (non-backbone) area through which the virtual link runs
(referred to as the virtual link's Transit area). Virtual links
cannot be configured through stub areas.

C.5 NBMA network parameters

OSPF treats an NBMA network much like it treats a broadcast
network. Since there may be many routers attached to the
network, a Designated Router is selected for the network. This
Designated Router then originates a network-LSA, which lists all
routers attached to the NBMA network.

However, due to the lack of broadcast capabilities, it may be
necessary to use configuration parameters in the Designated
Router selection. These parameters will only need to be
configured in those routers that are themselves eligible to
become Designated Router (i.e., those router's whose Router
Priority for the network is non-zero), and then only if no
automatic procedure for discovering neighbors exists:

List of all other attached routers
The list of all other routers attached to the NBMA network.
Each router is listed by its IP interface address on the
network. Also, for each router listed, that router's
eligibility to become Designated Router must be defined.
When an interface to a NBMA network comes up, the router
sends Hello Packets only to those neighbors eligible to
become Designated Router, until the identity of the
Designated Router is discovered.

PollInterval
If a neighboring router has become inactive (Hello Packets
have not been seen for RouterDeadInterval seconds), it may
still be necessary to send Hello Packets to the dead
neighbor. These Hello Packets will be sent at the reduced
rate PollInterval, which should be much larger than
HelloInterval. Sample value for a PDN X.25 network: 2
minutes.

C.6 Point-to-MultiPoint network parameters

On Point-to-MultiPoint networks, it may be necessary to
configure the set of neighbors that are directly reachable over
the Point-to-MultiPoint network. Each neighbor is identified by
its IP address on the Point-to-MultiPoint network. Designated
Routers are not elected on Point-to-MultiPoint networks, so the
Designated Router eligibility of configured neighbors is
undefined.

Alternatively, neighbors on Point-to-MultiPoint networks may be
dynamically discovered by lower-level protocols such as Inverse
ARP ([Ref14]).

C.7 Host route parameters

Host routes are advertised in router-LSAs as stub networks with
mask 0xffffffff. They indicate either router interfaces to
point-to-point networks, looped router interfaces, or IP hosts
that are directly connected to the router (e.g., via a SLIP
line). For each host directly connected to the router, the
following items must be configured:

Host IP address
The IP address of the host.

Cost of link to host
The cost of sending a packet to the host, in terms of the
link state metric. However, since the host probably has
only a single connection to the internet, the actual
configured cost in many cases is unimportant (i.e., will
have no effect on routing).

Area ID
The OSPF area to which the host belongs.

D. Authentication

All OSPF protocol exchanges are authenticated. The OSPF packet
header (see Section A.3.1) includes an authentication type field,
and 64-bits of data for use by the appropriate authentication scheme
(determined by the type field).

The authentication type is configurable on a per-interface (or
equivalently, on a per-network/subnet) basis. Additional
authentication data is also configurable on a per-interface basis.

Authentication types 0, 1 and 2 are defined by this specification.
All other authentication types are reserved for definition by the
IANA (iana@ISI.EDU). The current list of authentication types is
described below in Table 20.

AuType Description
___________________________________________
0 Null authentication
1 Simple password
2 Cryptographic authentication
All others Reserved for assignment by the
IANA (iana@ISI.EDU)

Table 20: OSPF authentication types.

D.1 Null authentication

Use of this authentication type means that routing exchanges
over the network/subnet are not authenticated. The 64-bit
authentication field in the OSPF header can contain anything; it
is not examined on packet reception. When employing Null
authentication, the entire contents of each OSPF packet (other
than the 64-bit authentication field) are checksummed in order
to detect data corruption.

D.2 Simple password authentication

Using this authentication type, a 64-bit field is configured on
a per-network basis. All packets sent on a particular network
must have this configured value in their OSPF header 64-bit
authentication field. This essentially serves as a "clear" 64-
bit password. In addition, the entire contents of each OSPF
packet (other than the 64-bit authentication field) are
checksummed in order to detect data corruption.

Simple password authentication guards against routers
inadvertently joining the routing domain; each router must first
be configured with its attached networks' passwords before it
can participate in routing. However, simple password
authentication is vulnerable to passive attacks currently
widespread in the Internet (see [Ref16]). Anyone with physical
access to the network can learn the password and compromise the
security of the OSPF routing domain.

D.3 Cryptographic authentication

Using this authentication type, a shared secret key is
configured in all routers attached to a common network/subnet.
For each OSPF protocol packet, the key is used to
generate/verify a "message digest" that is appended to the end
of the OSPF packet. The message digest is a one-way function of
the OSPF protocol packet and the secret key. Since the secret
key is never sent over the network in the clear, protection is
provided against passive attacks.

The algorithms used to generate and verify the message digest
are specified implicitly by the secret key. This specification
completely defines the use of OSPF Cryptographic authentication
when the MD5 algorithm is used.

In addition, a non-decreasing sequence number is included in
each OSPF protocol packet to protect against replay attacks.
This provides long term protection; however, it is still
possible to replay an OSPF packet until the sequence number
changes. To implement this feature, each neighbor data structure
contains a new field called the "cryptographic sequence number".
This field is initialized to zero, and is also set to zero

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 Key ID Auth Data Len
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Cryptographic sequence number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 18: Usage of the Authentication field
in the OSPF packet header when Cryptographic
Authentication is employed

whenever the neighbor's state transitions to "Down". Whenever an
OSPF packet is accepted as authentic, the cryptographic sequence
number is set to the received packet's sequence number.

This specification does not provide a rollover procedure for the
cryptographic sequence number. When the cryptographic sequence
number that the router is sending hits the maximum value, the
router should reset the cryptographic sequence number that it is
sending back to 0. After this is done, the router's neighbors
will reject the router's OSPF packets for a period of
RouterDeadInterval, and then the router will be forced to
reestablish all adjacencies over the interface. However, it is
expected that many implementations will use "seconds since
reboot" (or "seconds since 1960", etc.) as the cryptographic
sequence number. Such a choice will essentially prevent
rollover, since the cryptographic sequence number field is 32
bits in length.

The OSPF Cryptographic authentication option does not provide
confidentiality.

When cryptographic authentication is used, the 64-bit
Authentication field in the standard OSPF packet header is
redefined as shown in Figure 18. The new field definitions are
as follows:

Key ID
This field identifies the algorithm and secret key used to
create the message digest appended to the OSPF packet. Key
Identifiers are unique per-interface (or equivalently, per-
subnet).

Auth Data Len
The length in bytes of the message digest appended to the
OSPF packet.

Cryptographic sequence number
An unsigned 32-bit non-decreasing sequence number. Used to
guard against replay attacks.

The message digest appended to the OSPF packet is not actually
considered part of the OSPF protocol packet: the message digest
is not included in the OSPF header's packet length, although it
is included in the packet's IP header length field.

Each key is identified by the combination of interface and Key
ID. An interface may have multiple keys active at any one time.
This enables smooth transition from one key to another. Each key
has four time constants associated with it. These time constants
can be expressed in terms of a time-of-day clock, or in terms of
a router's local clock (e.g., number of seconds since last
reboot):

KeyStartAccept
The time that the router will start accepting packets that
have been created with the given key.

KeyStartGenerate
The time that the router will start using the key for packet
generation.

KeyStopGenerate
The time that the router will stop using the key for packet
generation.

KeyStopAccept
The time that the router will stop accepting packets that
have been created with the given key.

In order to achieve smooth key transition, KeyStartAccept should
be less than KeyStartGenerate and KeyStopGenerate should be less
than KeyStopAccept. If KeyStopGenerate and KeyStopAccept are
left unspecified, the key's lifetime is infinite. When a new key
replaces an old, the KeyStartGenerate time for the new key must
be less than or equal to the KeyStopGenerate time of the old
key.

Key storage should persist across a system restart, warm or
cold, to avoid operational issues. In the event that the last
key associated with an interface expires, it is unacceptable to
revert to an unauthenticated condition, and not advisable to
disrupt routing. Therefore, the router should send a "last
authentication key expiration" notification to the network
manager and treat the key as having an infinite lifetime until
the lifetime is extended, the key is deleted by network
management, or a new key is configured.

D.4 Message generation

After building the contents of an OSPF packet, the
authentication procedure indicated by the sending interface's
Autype value is called before the packet is sent. The
authentication procedure modifies the OSPF packet as follows.

D.4.1 Generating Null authentication

When using Null authentication, the packet is modified as
follows:

(1) The Autype field in the standard OSPF header is set to
0.

(2) The checksum field in the standard OSPF header is set to
the standard IP checksum of the entire contents of the
packet, starting with the OSPF packet header but
excluding the 64-bit authentication field. This
checksum is calculated as the 16-bit one's complement of
the one's complement sum of all the 16-bit words in the
packet, excepting the authentication field. If the

packet's length is not an integral number of 16-bit
words, the packet is padded with a byte of zero before
checksumming.

D.4.2 Generating Simple password authentication

When using Simple password authentication, the packet is
modified as follows:

(1) The Autype field in the standard OSPF header is set to
1.

(2) The checksum field in the standard OSPF header is set to
the standard IP checksum of the entire contents of the
packet, starting with the OSPF packet header but
excluding the 64-bit authentication field. This
checksum is calculated as the 16-bit one's complement of
the one's complement sum of all the 16-bit words in the
packet, excepting the authentication field. If the
packet's length is not an integral number of 16-bit
words, the packet is padded with a byte of zero before
checksumming.

(3) The 64-bit authentication field in the OSPF packet
header is set to the 64-bit password (i.e.,
authentication key) that has been configured for the
interface.

D.4.3 Generating Cryptographic authentication

When using Cryptographic authentication, there may be
multiple keys configured for the interface. In this case,
among the keys that are valid for message generation (i.e,
that have KeyStartGenerate <= current time <
KeyStopGenerate) choose the one with the most recent
KeyStartGenerate time. Using this key, modify the packet as
follows:

(1) The Autype field in the standard OSPF header is set to
2.

(2) The checksum field in the standard OSPF header is not
calculated, but is instead set to 0.

(3) The Key ID (see Figure 18) is set to the chosen key's
Key ID.

(4) The Auth Data Len field is set to the length in bytes of
the message digest that will be appended to the OSPF
packet. When using MD5 as the authentication algorithm,
Auth Data Len will be 16.

(5) The 32-bit Cryptographic sequence number (see Figure 18)
is set to a non-decreasing value (i.e., a value at least
as large as the last value sent out the interface). The
precise values to use in the cryptographic sequence
number field are implementation-specific. For example,
it may be based on a simple counter, or be based on the
system's clock.

(6) The message digest is then calculated and appended to
the OSPF packet. The authentication algorithm to be
used in calculating the digest is indicated by the key
itself. Input to the authentication algorithm consists
of the OSPF packet and the secret key. When using MD5 as
the authentication algorithm, the message digest
calculation proceeds as follows:

(a) The 16 byte MD5 key is appended to the OSPF packet.

(b) Trailing pad and length fields are added, as
specified in [Ref17].

(c) The MD5 authentication algorithm is run over the
concatenation of the OSPF packet, secret key, pad
and length fields, producing a 16 byte message
digest (see [Ref17]).

(d) The MD5 digest is written over the OSPF key (i.e.,
appended to the original OSPF packet). The digest is
not counted in the OSPF packet's length field, but

is included in the packet's IP length field. Any
trailing pad or length fields beyond the digest are
not counted or transmitted.

D.5 Message verification

When an OSPF packet has been received on an interface, it must
be authenticated. The authentication procedure is indicated by
the setting of Autype in the standard OSPF packet header, which
matches the setting of Autype for the receiving OSPF interface.

If an OSPF protocol packet is accepted as authentic, processing
of the packet continues as specified in Section 8.2. Packets
which fail authentication are discarded.

D.5.1 Verifying Null authentication

When using Null authentication, the checksum field in the
OSPF header must be verified. It must be set to the 16-bit
one's complement of the one's complement sum of all the 16-
bit words in the packet, excepting the authentication field.
(If the packet's length is not an integral number of 16-bit
words, the packet is padded with a byte of zero before
checksumming.)

D.5.2 Verifying Simple password authentication

When using Simple password authentication, the received OSPF
packet is authenticated as follows:

(1) The checksum field in the OSPF header must be verified.
It must be set to the 16-bit one's complement of the
one's complement sum of all the 16-bit words in the
packet, excepting the authentication field. (If the
packet's length is not an integral number of 16-bit
words, the packet is padded with a byte of zero before
checksumming.)

(2) The 64-bit authentication field in the OSPF packet
header must be equal to the 64-bit password (i.e.,
authentication key) that has been configured for the
interface.

D.5.3 Verifying Cryptographic authentication

When using Cryptographic authentication, the received OSPF
packet is authenticated as follows:

(1) Locate the receiving interface's configured key having
Key ID equal to that specified in the received OSPF
packet (see Figure 18). If the key is not found, or if
the key is not valid for reception (i.e., current time <
KeyStartAccept or current time >= KeyStopAccept), the
OSPF packet is discarded.

(2) If the cryptographic sequence number found in the OSPF
header (see Figure 18) is less than the cryptographic
sequence number recorded in the sending neighbor's data
structure, the OSPF packet is discarded.

(3) Verify the appended message digest in the following
steps:

(a) The received digest is set aside.

(b) A new digest is calculated, as specified in Step 6
of Section D.4.3.

(c) The calculated and received digests are compared. If
they do not match, the OSPF packet is discarded. If
they do match, the OSPF protocol packet is accepted
as authentic, and the "cryptographic sequence
number" in the neighbor's data structure is set to
the sequence number found in the packet's OSPF
header.

E. An algorithm for assigning Link State IDs

The Link State ID in AS-external-LSAs and summary-LSAs is usually
set to the described network's IP address. However, if necessary one
or more of the network's host bits may be set in the Link State ID.
This allows the router to originate separate LSAs for networks
having the same address, yet different masks. Such networks can
occur in the presence of supernetting and subnet 0s (see [Ref10]).

This appendix gives one possible algorithm for setting the host bits
in Link State IDs. The choice of such an algorithm is a local
decision. Separate routers are free to use different algorithms,
since the only LSAs affected are the ones that the router itself
originates. The only requirement on the algorithms used is that the
network's IP address should be used as the Link State ID whenever
possible; this maximizes interoperability with OSPF implementations
predating RFC1583.

The algorithm below is stated for AS-external-LSAs. This is only
for clarity; the exact same algorithm can be used for summary-LSAs.
Suppose that the router wishes to originate an AS-external-LSA for a
network having address NA and mask NM1. The following steps are then
used to determine the LSA's Link State ID:

(1) Determine whether the router is already originating an AS-
external-LSA with Link State ID equal to NA (in such an LSA the
router itself will be listed as the LSA's Advertising Router).
If not, the Link State ID is set equal to NA and the algorithm
terminates. Otherwise,

(2) Obtain the network mask from the body of the already existing
AS-external-LSA. Call this mask NM2. There are then two cases:

o NM1 is longer (i.e., more specific) than NM2. In this case,
set the Link State ID in the new LSA to be the network
[NA,NM1] with all the host bits set (i.e., equal to NA or'ed
together with all the bits that are not set in NM1, which is
network [NA,NM1]'s broadcast address).

o NM2 is longer than NM1. In this case, change the existing
LSA (having Link State ID of NA) to reference the new
network [NA,NM1] by incrementing the sequence number,

changing the mask in the body to NM1 and inserting the cost
of the new network. Then originate a new LSA for the old
network [NA,NM2], with Link State ID equal to NA or'ed
together with the bits that are not set in NM2 (i.e.,
network [NA,NM2]'s broadcast address).

The above algorithm assumes that all masks are contiguous; this
ensures that when two networks have the same address, one mask is
more specific than the other. The algorithm also assumes that no
network exists having an address equal to another network's
broadcast address. Given these two assumptions, the above algorithm
always produces unique Link State IDs. The above algorithm can also
be reworded as follows: When originating an AS-external-LSA, try to
use the network number as the Link State ID. If that produces a
conflict, examine the two networks in conflict. One will be a subset
of the other. For the less specific network, use the network number
as the Link State ID and for the more specific use the network's
broadcast address instead (i.e., flip all the "host" bits to 1). If
the most specific network was originated first, this will cause you
to originate two LSAs at once.

As an example of the algorithm, consider its operation when the
following sequence of events occurs in a single router (Router A).

(1) Router A wants to originate an AS-external-LSA for
[10.0.0.0,255.255.255.0]:

(a) A Link State ID of 10.0.0.0 is used.

(2) Router A then wants to originate an AS-external-LSA for
[10.0.0.0,255.255.0.0]:

(a) The LSA for [10.0.0,0,255.255.255.0] is reoriginated using a
new Link State ID of 10.0.0.255.

(b) A Link State ID of 10.0.0.0 is used for
[10.0.0.0,255.255.0.0].

(3) Router A then wants to originate an AS-external-LSA for
[10.0.0.0,255.0.0.0]:

(a) The LSA for [10.0.0.0,255.255.0.0] is reoriginated using a
new Link State ID of 10.0.255.255.

(b) A Link State ID of 10.0.0.0 is used for
[10.0.0.0,255.0.0.0].

(c) The network [10.0.0.0,255.255.255.0] keeps its Link State ID
of 10.0.0.255.

F. Multiple interfaces to the same network/subnet

There are at least two ways to support multiple physical interfaces
to the same IP subnet. Both methods will interoperate with
implementations of RFC1583 (and of course this memo). The two
methods are sketched briefly below. An assumption has been made that
each interface has been assigned a separate IP address (otherwise,
support for multiple interfaces is more of a link-level or ARP issue
than an OSPF issue).

Method 1:
Run the entire OSPF functionality over both interfaces, sending
and receiving hellos, flooding, supporting separate interface
and neighbor FSMs for each interface, etc. When doing this all
other routers on the subnet will treat the two interfaces as
separate neighbors, since neighbors are identified (on broadcast
and NBMA networks) by their IP address.

Method 1 has the following disadvantages:

(1) You increase the total number of neighbors and adjacencies.

(2) You lose the bidirectionality test on both interfaces, since
bidirectionality is based on Router ID.

(3) You have to consider both interfaces together during the
Designated Router election, since if you declare both to be
DR simultaneously you can confuse the tie-breaker (which is
Router ID).

Method 2:
Run OSPF over only one interface (call it the primary
interface), but include both the primary and secondary
interfaces in your Router-LSA.

Method 2 has the following disadvantages:

(1) You lose the bidirectionality test on the secondary
interface.

(2) When the primary interface fails, you need to promote the
secondary interface to primary status.

G. Differences from RFC2178

This section documents the differences between this memo and RFC
2178. All differences are backward-compatible. Implementations of
this memo and of RFCs 2178, 1583, and 1247 will interoperate.

G.1 Flooding modifications

Three changes have been made to the flooding procedure in
Section 13.

The first change is to step 4 in Section 13. Now MaxAge LSAs are
acknowledged and then discarded only when both a) there is no
database copy of the LSA and b) none of router's neighbors are
in states Exchange or Loading. In all other cases, the MaxAge
LSA is processed like any other LSA, installing the LSA in the
database and flooding it out the appropriate interfaces when the
LSA is more recent than the database copy (Step 5 of Section
13). This change also affects the contents of Table 19.

The second change is to step 5a in Section 13. The MinLSArrival
check is meant only for LSAs received during flooding, and
should not be performed on those LSAs that the router itself
originates.

The third change is to step 8 in Section 13. Confusion between
routers as to which LSA instance is more recent can cause a
disastrous amount of flooding in a link-state protocol (see
[Ref26]). OSPF guards against this problem in two ways: a) the
LS age field is used like a TTL field in flooding, to eventually
remove looping LSAs from the network (see Section 13.3), and b)
routers refuse to accept LSA updates more frequently than once
every MinLSArrival seconds (see Section 13). However, there is
still one case in RFC2178 where disagreements regarding which
LSA is more recent can cause a lot of flooding traffic:
responding to old LSAs by reflooding the database copy. For
this reason, Step 8 of Section 13 has been amended to only
respond with the database copy when that copy has not been sent
in any Link State Update within the last MinLSArrival seconds.

G.2 Changes to external path preferences

There is still the possibility of a routing loop in RFC2178
when both a) virtual links are in use and b) the same external
route is being imported by multiple ASBRs, each of which is in a
separate area. To fix this problem, Section 16.4.1 has been
revised. To choose the correct ASBR/forwarding address, intra-
area paths through non-backbone areas are always preferred.
However, intra-area paths through the backbone area (Area 0) and
inter-area paths are now of equal preference, and must be
compared solely based on cost.

The reasoning behind this change is as follows. When virtual
links are in use, an intra-area backbone path for one router can
turn into an inter-area path in a router several hops closer to
the destination. Hence, intra-area backbone paths and inter-area
paths must be of equal preference. We can safely compare their
costs, preferring the path with the smallest cost, due to the
calculations in Section 16.3.

Thanks to Michael Briggs and Jeremy McCooey of the UNH
InterOperability Lab for pointing out this problem.

G.3 Incomplete resolution of virtual next hops

One of the functions of the calculation in Section 16.3 is to
determine the actual next hop(s) for those destinations whose
next hop was calculated as a virtual link in Sections 16.1 and
16.2. After completion of the calculation in Section 16.3, any
paths calculated in Sections 16.1 and 16.2 that still have
unresolved virtual next hops should be discarded.

G.4 Routing table lookup

The routing table lookup algorithm in Section 11.1 has been
modified to reflect current practice. The "best match" routing
table entry is now always selected to be the one providing the
most specific (longest) match. Suppose for example a router is
forwarding packets to the destination 192.9.1.1. A routing table
entry for 192.9.1/24 will always be a better match than the
routing table entry for 192.9/16, regardless of the routing
table entries' path-types. Note however that when multiple paths

are available for a given routing table entry, the calculations
in Sections 16.1, 16.2, and 16.4 always yield the paths having
the most preferential path-type. (Intra-area paths are the most
preferred, followed in order by inter-area, type 1 external and
type 2 external paths; see Section 11).

Security Considerations

All OSPF protocol exchanges are authenticated. OSPF supports
multiple types of authentication; the type of authentication in use
can be configured on a per network segment basis. One of OSPF's
authentication types, namely the Cryptographic authentication
option, is believed to be secure against passive attacks and provide
significant protection against active attacks. When using the
Cryptographic authentication option, each router appends a "message
digest" to its transmitted OSPF packets. Receivers then use the
shared secret key and received digest to verify that each received
OSPF packet is authentic.

The quality of the security provided by the Cryptographic
authentication option depends completely on the strength of the
message digest algorithm (MD5 is currently the only message digest
algorithm specified), the strength of the key being used, and the
correct implementation of the security mechanism in all
communicating OSPF implementations. It also requires that all
parties maintain the secrecy of the shared secret key.

None of the OSPF authentication types provide confidentiality. Nor
do they protect against traffic analysis. Key management is also not
addressed by this memo.

For more information, see Sections 8.1, 8.2, and Appendix D.

Author's Address

John Moy
Ascend Communications, Inc.
1 Robbins Road
Westford, MA 01886

Phone: 978-952-1367
Fax: 978-392-2075
EMail: jmoy@casc.com

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