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Network Working Group                                           R. BalayRequest for Comments: 2973                         CoSine CommunicationsCategory: Informational                                          D. Katz                                                        Juniper Networks                                                               J. Parker                                                       Axiowave Networks                                                            October 2000IS-IS Mesh GroupsStatus 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 Contents1. Overview.....................................................22. Definitions of Mesh Groups...................................33. Drawbacks of Mesh Groups.....................................54. Interoperation with Mesh Groups..............................65. Acknowledgments..............................................66. References...................................................67. Security Considerations......................................68. Authors' Addresses...........................................79. Full Copyright Statement.....................................8Balay, et al.                Informational                      [Page 1]

RFC 2973                   IS-IS Mesh Groups                October 20001.  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   tosection 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: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 insection7.3.15.3 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   ofSection 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 ofSection 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 20003.  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.Balay, et al.                Informational                      [Page 5]

RFC 2973                   IS-IS Mesh Groups                October 20004.  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.Balay, et al.                Informational                      [Page 6]

RFC 2973                   IS-IS Mesh Groups                October 20008.  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.comBalay, et al.                Informational                      [Page 7]

RFC 2973                   IS-IS Mesh Groups                October 20009.  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   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   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Balay, et al.                Informational                      [Page 8]