Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


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.


 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] RFC 2973 IS-IS Mesh Groups October 2000

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

Balay, et al. Informational [Page 3] RFC 2973 IS-IS Mesh Groups October 2000

 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 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.

Balay, et al. Informational [Page 4] RFC 2973 IS-IS Mesh Groups October 2000

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
 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
 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.

Balay, et al. Informational [Page 5] RFC 2973 IS-IS Mesh Groups October 2000

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

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.

Balay, et al. Informational [Page 6] RFC 2973 IS-IS Mesh Groups October 2000

8. Authors' Addresses

 Rajesh Balay
 CoSine Communications, Inc
 1200 Bridge Parkway
 Redwood City, CA 94065
 Dave Katz
 Juniper Networks
 385 Ravendale Drive
 Mountain View, CA 94043
 Jeff Parker
 Axiowave Networks,
 100 Nickerson Road,
 Marlborough, MA 01752

Balay, et al. Informational [Page 7] RFC 2973 IS-IS Mesh Groups October 2000

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
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 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


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

Balay, et al. Informational [Page 8]

/data/webs/external/dokuwiki/data/pages/rfc/rfc2973.txt · Last modified: 2000/10/06 19:14 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki