Network Working Group A. Zinin
Request for Comments: 3509 Alcatel
Category: Informational A. Lindem
Redback Networks
D. Yeung
Procket Networks
April 2003

Alternative Implementations of OSPF Area Border Routers

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

Open Shortest Path First (OSPF) is a link-state intra-domain routing
protocol used for routing in IP networks. Though the definition of
the Area Border Router (ABR) in the OSPF specification does not
require a router with multiple attached areas to have a backbone
connection, it is actually necessary to provide sUCcessful routing to
the inter-area and external destinations. If this requirement is not
met, all traffic destined for the areas not connected to such an ABR
or out of the OSPF domain, is dropped. This document describes
alternative ABR behaviors implemented in Cisco and IBM routers.

1 Overview

1.1 Introduction

An OSPF routing domain can be split into several subdomains, called
areas, which limit the scope of LSA flooding. According to [Ref1] a
router having attachments to multiple areas is called an "area border
router" (ABR). The primary function of an ABR is to provide its
attached areas with Type-3 and Type-4 LSAs, which are used for
describing routes and AS boundary routers (ASBRs) in other areas, as
well as to perform actual inter-area routing.

1.2 Motivation

In OSPF domains the area topology is restricted so that there must be
a backbone area (area 0) and all other areas must have either
physical or virtual connections to the backbone. The reason for this
star-like topology is that OSPF inter-area routing uses the
distance-vector approach and a strict area hierarchy permits
avoidance of the "counting to infinity" problem. OSPF prevents
inter-area routing loops by implementing a split-horizon mechanism,
allowing ABRs to inject into the backbone only Summary-LSAs derived
from the
intra-area routes, and limiting ABRs' SPF calculation to consider
only Summary-LSAs in the backbone area's link-state database.

The last restriction leads to a problem when an ABR has no backbone
connection (in OSPF, an ABR does not need to be attached to the
backbone). Consider a sample OSPF domain depicted in the Figure 1.

. .
. Area 0 .
+--+ +--+
..R1.. ..R2..
. +--+ .. +--+ .
. .. .
. +--+ .
. Area1 R3 Area2 .
. +--+ +--+ .
. .. R4 .
. . . +--+ .
....... .......

Figure 1. ABR dropping transit traffic

In this example R1, R2, and R3 are ABRs. R1 and R2 have backbone
connections, while R3 doesn't.

Following the section 12.4.1 of [Ref1], R3 will identify itself as an
ABR by setting the bit B in its router-LSA. Being an ABR, R3 can
only consider summary-LSAs from the backbone when building the
routing table (according to section 16.2 of [Ref1]), so it will not
have any inter-area routes in its routing table, but only intra-area
routes from both Area 1 and Area 2. Consequently, according to
section 12.4.3 of [Ref1], R3 will originate into Areas 1 and 2 only
summary-LSAs covering destinations in the directly attached areas,
i.e., in Area 2---the summary-LSAs for Area 1, and in Area 1---the
summary-LSAs for Area 2.

At the same time, router R2, as an ABR connected to the backbone,
will inject into Area 2 summary-LSAs describing the destinations in
Area 0 (the backbone), Area 1 and other areas reachable through the
backbone.

This results in a situation where internal router R4 calculates its
routes to destinations in the backbone and areas other than Area 1
via R2. The topology of Area 2 itself can be such that the best path
from R4 to R2 is via R3, so all traffic destined for the backbone and
backbone-attached areas goes through R3. Router R3 in turn, having
only intra-area routes for areas 1 and 2, will drop all traffic not
destined for the areas directly attached to it. The same problem can
occur when a backbone-connected ABR loses all of its adjacencies in
the backbone---even if there are other normally functioning ABRs in
the attached areas, all traffic going to the backbone (destined for
it or for other areas) will be dropped.

In a standard OSPF implementation this situation can be remedied by
use of Virtual Links (see section 15 of [Ref1] for more details). In
this case, router R3 will have a virtual backbone connection, will
form an adjacency over it, will receive all LSAs directly from a
backbone-attached router (R1 or R2, or both in our example) and will
install intra- or inter-area routes.

While being an unavoidable technique for repairing a partitioned
backbone area, the use of virtual links in the described situation
adds extra configuration headaches and system traffic overhead.

Another situation where standard ABR behavior does not provide
acceptable results is when it is necessary to provide redundant
connectivity to an ASBR via several different OSPF areas. This would
allow a provider to aggregate all their customers connecting through
a single Access point into one area while still offering a redundant
connection through another access point in a different area, as shown
in Figure 2.

. .
. Area 0 .
+--+ +--+
..R1.. ..R2..
. +--+ .. +--+ .
. .. .
. .. .
. Area1 .. Area2 .
. .. .
. .. .
. +--+ .
.......R3.......
ASBR+--+
/..
--+- -+--
CN1 CNx

Customer Networks (CN1--CNx) Advertised
as AS External or NSSA External Routes

Figure 2. Dual Homed Customer Router

This technique is already used in a number of networks including one
of a major provider.

The next section describes alternative ABR behaviors, implemented in
Cisco and IBM routers. The changes are in the ABR definition and
inter-area route calculation. Any other parts of standard OSPF are
not changed.

These solutions are targeted to the situation when an ABR has no
backbone connection. They imply that a router connected to multiple
areas without a backbone connection is not an ABR and should function
as a router internal to every attached area. This solution emulates
a situation where separate OSPF processes are run for each area and
supply routes to the routing table. It remedies the situation
described in the examples above by not dropping transit traffic.
Note that a router following it does not function as a real border
router---it doesn't originate summary-LSAs. Nevertheless such a
behavior may be desirable in certain situations.

Note that the proposed solutions do not obviate the need of virtual
link configuration in case an area has no physical backbone
connection at all. The methods described here improve the behavior
of a router connecting two or more backbone-attached areas.

2 Changes to ABR Behavior

2.1 Definitions

The following definitions will be used in this document to describe
the new ABR behaviors:

Configured area:
An area is considered configured if the router has at least one
interface in any state assigned to that area.

Actively Attached area:
An area is considered actively attached if the router has at least
one interface in that area in the state other than Down.

Active Backbone Connection:
A router is considered to have an active backbone connection if
the backbone area is actively attached and there is at least one
fully adjacent neighbor in it.

Area Border Router (ABR):

Cisco Systems Interpretation:
A router is considered to be an ABR if it has more than one
area Actively Attached and one of them is the backbone area.

IBM Interpretation:
A router is considered to be an ABR if it has more than one
Actively Attached area and the backbone area Configured.

2.2 Implementation Details

The following changes are made to the base OSPF, described in [Ref1]:

1. The algorithm for Type 1 LSA (router-LSA) origination is changed
to prevent a multi-area connected router from identifying itself
as an ABR by the bit B (as described in section 12.4.1 of [Ref1])
until it considers itself as an ABR according to the definitions
given in section 2.1.

2. The algorithm for the routing table calculation is changed to
allow the router to consider the summary-LSAs from all attached
areas if it is not an ABR, but has more than one attached area,
or it does not have an Active Backbone Connection. Definitions
of the terms used in this paragraph are given in section 2.1.

So, the paragraph 1 of section 16.2 of [Ref1] should be
interpreted as follows:

"The inter-area routes are calculated by examining summary-LSAs.
If the router is an ABR and has an Active Backbone Connection,
only backbone summary-LSAs are examined. Otherwise (either the
router is not an ABR or it has no Active Backbone Connection),
the router should consider summary-LSAs from all Actively
Attached areas..."

3. For Cisco ABR approach, the algorithm for the summary-LSAs
origination is changed to prevent loops of summary-LSAs in
situations where the router considers itself an ABR but doesn't
have an Active Backbone Connection (and, consequently, examines
summaries from all attached areas). The algorithm is changed to
allow an ABR to announce only intra-area routes in such a
situation.

So, the paragraph 2 of subsection 12.4.3 of [Ref1] should be
interpreted as follows:

"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 while
only intra-area routes are advertised into the backbone, if the
router has an Active Backbone Connection, both intra-area and
inter-area routes are advertised into the other areas; otherwise,
the router only advertises intra-area routes into non-backbone
areas."

For this policy to be applied we change steps 6 and 7 in the
summary origination algorithm to be as follows:

Step 6:

"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. If the ABR
performing this algorithm does not have an Active Backbone
Connection, it can originate Type 4 summary-LSA only if the
type of the route to the ASBR is intra-area. Note: Type 4
summary-LSAs should not be generated if Area A has been
configured as a stub area."

Step 7:

"Else, the Destination type is network. If this is an
inter-area route and the ABR performing this algorithm has an
Active Backbone Connection, 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."

The changes in the ABR behavior described in this section allow a
multi-area connected router to successfully route traffic destined
for the backbone and other areas. Note that if the router does not
have a backbone area Configured it does not actively attract
inter-area traffic, because it does not consider itself an ABR and
does not originate summary-LSAs. It still can forward traffic from
one attached area to another along intra-area routes in case other
routers in corresponding areas have the best inter-area paths over
it, as described in section 1.2.

By processing all summaries when the backbone is not active, we
prevent the ABR, which has just lost its last backbone adjacency,
from dropping any packets going through the ABR in question to
another ABR and destined towards the backbone or other areas not
connected to the ABR directly.

3 Virtual Link Treatment

The Cisco ABR approach described in this document requires an ABR to
have at least one active interface in the backbone area. This
requirement may cause problems with virtual links in those rare
situations where the backbone area is purely virtual, as shown in
Figure 3, and the state of the VL is determined as in [Ref1].

....... ........... ......
. . . .
+--+ VL +--+
R1***********R2
+--+ +--+
Area 1 . . Area 2 . . Area 3
....... ........... ......

Figure 3. Purely Virtual Backbone

If R1 and R2 treat virtual links as in [Ref1], their virtual links
will never go up, because their router-LSAs do not contain the B-bit,
which is, in turn, because the routers do not have active interfaces
(virtual links) in the backbone and do not consider themselves ABRs.

Note that this problem does not appear if one of the routers has a
real interface in the backbone, as it usually is in real networks.

Though the situation described is deemed to be rather rare,
implementations supporting Cisco ABR behavior may consider changing
VL-specific code so that a virtual link is reported up (an
InterfaceUp event is generated) when a router with corresponding
router-ID is seen via Dijkstra, no matter whether its router-LSA
indicates that it is an ABR or not. This means that checking of
configured virtual links should be done not in step 4 of the
algorithm in 16.1 of [Ref1] when a router routing entry is added, but
every time a vertex is added to the SPT in step 3 of the same
algorithm.

4 Compatibility

The changes of the OSPF ABR operations do not influence any ASPects
of the router-to-router cooperation and do not create routing loops,
and hence are fully compatible with standard OSPF. Proof of
compatibility is outside the scope of this document.

5 Deployment Considerations

This section discusses the deployments details of the ABR behaviors
described in this document. Note that this approach is fully
compatible with standard ABR behavior, so ABRs acting as described in
[Ref1] and in this document can coexist in an OSPF domain and will
function without problems.

Deployment of ABRs using the alternative methods improves the
behavior of a router connected to multiple areas without a backbone
attachment, but can lead to uneXPected routing asymmetry, as
described below.

Consider an OSPF domain depicted in Figure 4.

. Backbone .
. .
. --------------------- .
. 1 1 .
..+--+.............+--+..
..R1..... ....R4..
. +--+ . . +--+ .
. 1 . . /4 .
. 8 +--+ 4 / .
. +-R3---+ .
. 1 / +--+4 .
. +--+ / . . 4 +--+ .
. R2/8 . . +--R5 .
. +--+ . . +--+ .
. . . .
. --------- . . -------- .
. net N . . net M .
. . . .
. Area 1 . . Area 2 .
........... ..........

Figure 4. Inter-area routing asymmetry

Assume that R3 uses the approach described in this document. In this
case R2 will have inter-area routes to network M via ABR R1 only. R5
in turn will have its inter-area route to network N via R4, but as
far as R4 is only reachable via R3, all traffic destined to network N
will pass through R3. R3 will have an intra-area route to network N
via R2 and will, of course, route it directly to it (because
intra-area routes are always preferred over inter-area ones).
Traffic going back from network N to network M will pass through R2
and will be routed to R1, as R2 will not have any inter-area routes
via R3. So, traffic from N to M will always go through the backbone
while traffic from M to N will cross the areas directly via R3 and,
in this example, will not use a more optimal path through the
backbone.

Note that this problem is not caused by the fact that R3 uses the
alternative approach. The reason for attracting the attention to it
is that R3 is not really functioning as an ABR in case this new
behavior is used, i.e., it does not inject summary-LSAs into the
attached areas, but inter-area traffic can still go through it.

6 Security Considerations

The alternative ABR behaviors specified in this document do not raise
any security issues that are not already covered in [Ref1].

7 Acknowledgements

Authors would like to thank Alvaro Retana, Russ White, and Liem
Nguyen for their review of the document.

8 Disclaimer

This document describes OSPF ABR implementations of respective
vendors "as is", only for informational purposes, and without any
warranties, guarantees or support. These implementations are subject
to possible future changes. For the purposes of easier deployment,
information about software versions where described behavior was
integrated is provided below.

Initial Cisco ABR implementation (slightly different from the one
described in this memo, requiring non-backbone areas to be
configured, and not necessarily actively attached in the ABR
definition) was introduced in Cisco IOS (tm) version 11.1(6). Cisco
ABR behavior described in this document was integrated in Cisco IOS
(tm) in version 12.1(3)T.

The ABR behavior described as IBM ABR approach was implemented by IBM
in IBM Nways Multiprotocol Routing Services (MRS) 3.3.

Note that the authors do not intend to keep this document in sync
with actual implementations.

10 References

[Ref1] Moy, J., "OSPF version 2", STD 54, RFC2328, April 1998.

11 Authors' Addresses

Alex Zinin
Alcatel

EMail: zinin@psg.com

Derek M. Yeung
Procket Networks
1100 Cadillac Ct
Milpitas, CA 95035

Phone: 408-635-7911
EMail: myeung@procket.com

Acee Lindem
Redback Networks
102 Carric Bend Court
Cary, NC 27519 USA

Phone: 919-387-6971
EMail: acee@redback.com

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