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RFC 2973 - IS-IS Mesh Groups


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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

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.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
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   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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