Network Working Group R. Balay
Request for Comments: 2973 CoSine Communications
Category: Informational D. Katz
Juniper Networks
J. Parker
Axiowave Networks
October 2000

IS-IS Mesh Groups

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 (2000). All Rights Reserved.

Abstract

This document describes a mechanism to redUCe redundant packet
transmissions for the Intermediate System to Intermediate System
(IS-IS) Routing protocol, as described in ISO 10589. The described
mechanism can be used to reduce the flooding of Link State PDUs
(Protocol Data Units) (LSPs) in IS-IS topologies. The net effect is
to engineer a flooding topology for LSPs which is a subset of the
physical topology. This document serves to document the existing
behavior in deployed implementations.

The document describes behaviors that are backwards compatible with
implementations that do not support this feature.

Table of Contents

1. Overview..................................................... 2
2. Definitions of Mesh Groups................................... 3
3. Drawbacks of Mesh Groups..................................... 5
4. Interoperation with Mesh Groups.............................. 6
5. Acknowledgments.............................................. 6
6. References................................................... 6
7. Security Considerations...................................... 6
8. Authors' Addresses........................................... 7
9. Full Copyright Statement..................................... 8

1. Overview

In ATM or frame relay networks Intermediate Systems are inter-
connected using virtual circuits (VCs) which are logical point-to-
point links. Some organizations attach multiple Intermediate Systems
to form a full "mesh" topology, where every pair of Intermediate
Systems are connected by a point-to-point link. In such topologies,
IS-IS protocol operation leads to redundant transmission of certain
PDUs due to the flooding operation which is illustrated below.

When an Intermediate System gets a new Link State Protocol Data Unit
(LSP), it stores it, and prepares to flood it out every circuit
except the source circuit. This is done by setting SRM (Send Routing
Message) bits held in the local copy of the LSP: there is an SRM for
each circuit.

+----------+ +----------+
I12 I21
System 1 --------------------------- System 2

+----------+ +----------+
I13 I14 I23 / I24
/
/
/
/
/
/
.
/
/
/
/
/
/
I31 / I32 I41 I42
+----------+ +----------+

System 3 --------------------------- System 4
I34 I43
+----------+ +----------+

Figure 1. A four node full mesh topology

When System1 regenerates an LSP, it will flood the LSP through the
network by marking the SRM bits for circuits I12, I14, and I13. In
due course, it will send out the LSP on each circuit.

When System2 receives System1's LSP, it propagates the PDU according
to section 7.2.14 of ISO 10589 [1]. It sets the SRM bits on circuits
I23 and I24, to flood the LSP to System3 and System4. However, these
Intermediate Systems will get the LSP directly from System1. In a
full mesh of N Intermediate Systems, the standard protocol mechanism
results in N-2 extra transmissions of each LSP, a waste of bandwidth
and processing effort, with little gain in reliability.

Mesh groups provide a mechanism to reduce the flooding of LSPs.

2. Definitions of Mesh Groups

A mesh group is defined as a set of point-to-point circuits which
provide full connectivity to a set of Intermediate Systems. Each
circuit has two new attributes: meshGroupEnabled, which is in state
{meshInactive, meshBlocked, or meshSet} and an integer variable
meshGroup, which is valid only if the value of meshGroupEnabled
attribute is 'meshSet'. Circuits that are in state 'meshSet' and
that have the same value of meshGroup are said to be in the same mesh
group.

LSPs are not flooded over circuits in 'meshBlocked' state, and an LSP
received on a circuit C is not flooded out circuits that belong to
C's mesh group.

Section 7.3.15.1 clause e.1.ii) of ISO 10589 [1] is modified as
follows:

e.1.ii)
if the meshGroupEnabled attribute is 'meshSet' for the
circuit C, set the SRMflag for that LSP for all circuits
other than C whose meshGroupEnabled attribute is
'meshInactive'. Also set the SRMflag for all circuits in
state 'meshSet' whose meshGroup attribute is not the same
as C's.

if the meshGroupEnabled attribute is 'meshInactive' for
circuit C, set the SRMflag for that LSP for all circuits
other than C whose meshGroupEnabled attribute is not
'meshBlocked'.

For robust database synchronization when using mesh groups, the
Complete Sequence Number PDUs (CSNPs) are sent periodically on
point-to-point links with a mesh group meshEnabled or meshBlocked.
Section 7.3.15.3 clause b) of ISO 10589 [1] is modified as follows:

b) If C is a point-to-point circuit (including non-DA DED
circuits and virtual links), then

1) If the circuit's attribute is 'meshSet' or 'meshBlocked',
then for each valid level, the IS will send a complete
set of CSNPs as described for a Designated IS in section
7.3.15.3 clause a).

2) CSNPs are transmitted only at initialization on point-
to-point links whose state is 'meshInactive'.

Use of mesh groups at an Intermediate System also modifies the
behavior in transmission of generated LSPs. These LSPs are not
required to be transmitted over circuits in state 'meshBlocked' at
system startup or when the LSP is regenerated. The second sentence
of Section 7.3.12 is modified to read:

"For all the circuits whose meshGroupEnabled attribute is
not 'meshBlocked', the IS shall set the SRMflags for that
Link State PDU to propagate it on all these circuits. The
IS shall clear the SRMflags for circuits whose
meshGroupEnabled attribute is 'meshBlocked'."

Some of the transient transmission overhead can be reduced by having
an Intermediate System not transmit its copies of the LSPs in
database on a circuit start-up/restart if the circuit is '
meshBlocked'. The clause a) in the last part of Section 7.3.17 of
ISO 10589, which refers to the point-to-point circuits, is modified
as follows:

a) set SRMflag for that circuit on all LSPs if the
meshGroupEnabled attribute of the circuit is not
'meshBlocked', and

Numbering of mesh groups provides the ability to divide a large full
mesh topology into a smaller group of full mesh sub-topologies (mesh
groups). These mesh groups are connected by "transit" circuits which
are 'meshInactive', while the remaining circuits between the mesh
groups are configured as 'meshBlocked' to reduce flooding redundancy.
Use of numbering makes mesh groups more scalable.

3. Drawbacks of Mesh Groups

The mesh group feature described in this document is a simple
mechanism to reduce flooding of LSPs in some IS-IS topologies. It
relies on a correct user configuration. If a combination of user
configuration and link failures result in a partitioned flooding
topology, LSPs will not be sent in a timely fashion, which may lead
to routing loops or black holes.

The concept of using numbered mesh groups also suffers from the
complexity and reliance on static configuration, making the
topologies brittle. Loosing a transit link can partition LSP
flooding in unpredictable ways, requiring the periodic flooding of
CSNPs to synchronize databases. In large networks, CSNPs become
large and also consume bandwidth.

The authors are not aware of any networks that have deployed numbered
mesh groups: instead, administrators set links to state 'meshBlocked'
to prune the flooding topology (also known as "poor man's mesh
groups").

Some improvements to mesh groups which have been suggested include:

a) To negotiate or check the mesh group attributes during
initialization of an adjacency to verify that the two ends of
every circuit hold identical values of the mesh state and mesh
number.

b) Dynamic election of active transit links so that a topology could
recover from failure of transit circuits.

c) Reduce the flooding of CSNPs by sending them periodically on some
meshGroup circuits rather than all circuits.

d) Reduce the size of PDUs required by flooding of CSNPs by sending
CSNP summaries: checksums or sequence numbers.

e) A related problem is the unneeded multiple transmissions of LSPs
to neighbors that are connected via multiple links. The protocol
could use the remote system ID of each adjacency and attempt to
send a single copy of each LSP to a neighbor.

Any such improvements are outside the scope of this document, and may
be the basis for future work.

4. Interoperation with Mesh Groups

Since mesh groups do not alter the content of packets, an
Intermediate System that does not implement mesh groups will not see
any different packets or new TLVs. The only impact will be that
additional CSNPs will be seen on some point-to-point links. A
conformant implementation can be eXPected to respond correctly to
extra CSNPs.

5. Acknowledgments

The original idea for mesh groups is due to Dave Katz. Thanks to
Tony Li, Tony Przygienda, Peter Livesey, and Henk Smit for helpful
comments.

6. References

[1] ISO/IEC 10589, "Intermediate System to Intermediate System
Intra-Domain Routing Exchange Protocol for use in Conjunction
with the Protocol for Providing the Connectionless-mode Network
Service (ISO 8473)", June 1992.

7. Security Considerations

This document raises no new security issues for IS-IS.

8. Authors' Addresses

Rajesh Balay
CoSine Communications, Inc
1200 Bridge Parkway
Redwood City, CA 94065

EMail: Rajesh.Balay@cosinecom.com

Dave Katz
Juniper Networks
385 Ravendale Drive
Mountain View, CA 94043

EMail: dkatz@juniper.net

Jeff Parker
Axiowave Networks,
100 Nickerson Road,
Marlborough, MA 01752

EMail: jparker@axiowave.com

9. Full Copyright Statement

Copyright (C) The Internet Society (2000). All Rights Reserved.

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