Internet Engineering Task Force R. Balay
INTERNET DRAFT Ericsson
D. Katz
Juniper Networks
J. Parker
Lucent Technology
July 7, 2000
IS-IS Mesh Groups
<draft-ietf-isis-wg-mesh-group-01.txt>
1. Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet
Engineering Task Force (IETF), its areas, and its working
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documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
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"work in progress."
The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed
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Copyright Notice
Copyright (C) The Internet Society (1997). All Rights
Reserved.
INTERNET DRAFT IS-IS Mesh Groups July 2000
2. Abstract
This document describes a mechanism to reduce redundant packet
transmissions for the IS-IS Routing protocol, as described in
ISO 10589 [1]. The described mechanism can be used to reduce
the flooding of Link State PDUs (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 draft serves
to document the existing behavior in deployed implementations.
The draft describes behaviors that are backwards compatible
with implementations that do not support this feature.
This document is provided to the IETF working group on IS-IS.
Table of Contents
1. Status of this Memo.................................. 1
2. Abstract............................................. 2
3. Overview............................................. 2
4. Definitions of Mesh Groups........................... 4
5. Drawbacks of Mesh Groups............................. 6
6. Interoperation with Mesh Groups...................... 7
7. Acknowledgments...................................... 7
8. References........................................... 7
9. Security Considerations.............................. 8
10. Authors' Address..................................... 8
11. Full Copyright Statement............................. 9
3. 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.
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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
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effort, with little gain in reliability.
Mesh groups provide a mechanism to reduce the flooding of
LSPs.
4. 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
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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
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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.
5. 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.
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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.
Any such improvements are outside the scope of this document,
and may be the basis for future work.
6. 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.
7. 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.
8. References
[1] ISO/IEC 10589, "Intermediate System to Intermediate
System Intra- Domain Routeing Exchange Protocol for use
in Conjunction with the Protocol for Providing the
Connectionless-mode Network Service (ISO 8473)", June
1992.
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9. Security Considerations
This document raises no new security issues for IS-IS.
10. Authors' Address
Rajesh Balay
CoSine Communications
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
Lucent Technologies,
200 Nickerson Road,
Marlborough, MA 01752
email: jparker@nexabit.com
11. Full Copyright Statement
Copyright (C) The Internet Society (1997). All Rights
Reserved.
This document and translations of it may be copied and
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for copyrights defined in the Internet Standards process must
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