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IS-IS Mesh Groups
RFC 2973

Document type: RFC - Informational (October 2000; No errata)
Document stream: IETF
Last updated: 2013-03-02
Other versions: plain text, pdf, html

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IESG State: RFC 2973 (Informational)
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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

Balay, et al.                Informational                      [Page 1]
RFC 2973                   IS-IS Mesh Groups                October 2000

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.

Balay, et al.                Informational                      [Page 2]

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