As per Relevance of the word circuits, we have this rfc below:
Network Working Group R.
Request for Comments: 2973 CoSine
Category: Informational D.
Juniper
J.
Axiowave
October 2000
IS-IS Mesh
Status of this
This memo provides information for the Internet community. It
not specify an Internet standard of any kind. Distribution of
memo is unlimited
Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document describes a mechanism to reduce redundant
transmissions for the Intermediate System to Intermediate
(IS-IS) Routing protocol, as described in ISO 10589. The
mechanism can be used to reduce the flooding of Link State
(Protocol Data Units) (LSPs) in IS-IS topologies. The net effect
to engineer a flooding topology for LSPs which is a subset of
physical topology. This document serves to document the
behavior in deployed implementations
The document describes behaviors that are backwards compatible
implementations that do not support this feature
Table of
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.
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
to form a full "mesh" topology, where every pair of
Systems are connected by a point-to-point link. In such topologies
IS-IS protocol operation leads to redundant transmission of
PDUs due to the flooding operation which is illustrated below
When an Intermediate System gets a new Link State Protocol Data
(LSP), it stores it, and prepares to flood it out every
except the source circuit. This is done by setting SRM (Send
Message) bits held in the local copy of the LSP: there is an SRM
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
When System1 regenerates an LSP, it will flood the LSP through
network by marking the SRM bits for circuits I12, I14, and I13.
due course, it will send out the LSP on each circuit
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RFC 2973 IS-IS Mesh Groups October 2000
When System2 receives System1's LSP, it propagates the PDU
to section 7.2.14 of ISO 10589 [1]. It sets the SRM bits on
I23 and I24, to flood the LSP to System3 and System4. However,
Intermediate Systems will get the LSP directly from System1. In
full mesh of N Intermediate Systems, the standard protocol
results in N-2 extra transmissions of each LSP, a waste of
and processing effort, with little gain in reliability
Mesh groups provide a mechanism to reduce the flooding of LSPs
2. Definitions of Mesh
A mesh group is defined as a set of point-to-point circuits
provide full connectivity to a set of Intermediate Systems.
circuit has two new attributes: meshGroupEnabled, which is in
{meshInactive, meshBlocked, or meshSet} and an integer
meshGroup, which is valid only if the value of
attribute is 'meshSet'. Circuits that are in state 'meshSet'
that have the same value of meshGroup are said to be in the same
group
LSPs are not flooded over circuits in 'meshBlocked' state, and an
received on a circuit C is not flooded out circuits that belong
C's mesh group
Section 7.3.15.1 clause e.1.ii) of ISO 10589 [1] is modified
follows
e.1.ii
if the meshGroupEnabled attribute is 'meshSet' for
circuit C, set the SRMflag for that LSP for all
other than C whose meshGroupEnabled attribute
'meshInactive'. Also set the SRMflag for all circuits
state 'meshSet' whose meshGroup attribute is not the
as C's
if the meshGroupEnabled attribute is 'meshInactive'
circuit C, set the SRMflag for that LSP for all
other than C whose meshGroupEnabled attribute is
'meshBlocked'.
For robust database synchronization when using mesh groups,
Complete Sequence Number PDUs (CSNPs) are sent periodically
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
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RFC 2973 IS-IS Mesh Groups October 2000
b) If C is a point-to-point circuit (including non-DA
circuits and virtual links),
1) If the circuit's attribute is 'meshSet' or 'meshBlocked',
then for each valid level, the IS will send a
set of CSNPs as described for a Designated IS in
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
behavior in transmission of generated LSPs. These LSPs are
required to be transmitted over circuits in state 'meshBlocked'
system startup or when the LSP is regenerated. The second
of Section 7.3.12 is modified to read
"For all the circuits whose meshGroupEnabled attribute
not 'meshBlocked', the IS shall set the SRMflags for
Link State PDU to propagate it on all these circuits.
IS shall clear the SRMflags for circuits
meshGroupEnabled attribute is 'meshBlocked'."
Some of the transient transmission overhead can be reduced by
an Intermediate System not transmit its copies of the LSPs
database on a circuit start-up/restart if the circuit is '
meshBlocked'. The clause a) in the last part of Section 7.3.17
ISO 10589, which refers to the point-to-point circuits, is
as follows
a) set SRMflag for that circuit on all LSPs if
meshGroupEnabled attribute of the circuit is
'meshBlocked',
Numbering of mesh groups provides the ability to divide a large
mesh topology into a smaller group of full mesh sub-topologies (
groups). These mesh groups are connected by "transit" circuits
are 'meshInactive', while the remaining circuits between the
groups are configured as 'meshBlocked' to reduce flooding redundancy
Use of numbering makes mesh groups more scalable
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RFC 2973 IS-IS Mesh Groups October 2000
3. Drawbacks of Mesh
The mesh group feature described in this document is a
mechanism to reduce flooding of LSPs in some IS-IS topologies.
relies on a correct user configuration. If a combination of
configuration and link failures result in a partitioned
topology, LSPs will not be sent in a timely fashion, which may
to routing loops or black holes
The concept of using numbered mesh groups also suffers from
complexity and reliance on static configuration, making
topologies brittle. Loosing a transit link can partition
flooding in unpredictable ways, requiring the periodic flooding
CSNPs to synchronize databases. In large networks, CSNPs
large and also consume bandwidth
The authors are not aware of any networks that have deployed
mesh groups: instead, administrators set links to state 'meshBlocked
to prune the flooding topology (also known as "poor man's
groups").
Some improvements to mesh groups which have been suggested include
a) To negotiate or check the mesh group attributes
initialization of an adjacency to verify that the two ends
every circuit hold identical values of the mesh state and
number
b) Dynamic election of active transit links so that a topology
recover from failure of transit circuits
c) Reduce the flooding of CSNPs by sending them periodically on
meshGroup circuits rather than all circuits
d) Reduce the size of PDUs required by flooding of CSNPs by
CSNP summaries: checksums or sequence numbers
e) A related problem is the unneeded multiple transmissions of
to neighbors that are connected via multiple links. The
could use the remote system ID of each adjacency and attempt
send a single copy of each LSP to a neighbor
Any such improvements are outside the scope of this document, and
be the basis for future work
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RFC 2973 IS-IS Mesh Groups October 2000
4. Interoperation with Mesh
Since mesh groups do not alter the content of packets,
Intermediate System that does not implement mesh groups will not
any different packets or new TLVs. The only impact will be
additional CSNPs will be seen on some point-to-point links.
conformant implementation can be expected to respond correctly
extra CSNPs
5.
The original idea for mesh groups is due to Dave Katz. Thanks
Tony Li, Tony Przygienda, Peter Livesey, and Henk Smit for
comments
6.
[1] ISO/IEC 10589, "Intermediate System to Intermediate
Intra-Domain Routing Exchange Protocol for use in
with the Protocol for Providing the Connectionless-mode
Service (ISO 8473)", June 1992.
7. Security
This document raises no new security issues for IS-IS
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RFC 2973 IS-IS Mesh Groups October 2000
8. Authors'
Rajesh
CoSine Communications,
1200 Bridge
Redwood City, CA 94065
EMail: Rajesh.Balay@cosinecom.
Dave
Juniper
385 Ravendale
Mountain View, CA 94043
EMail: dkatz@juniper.
Jeff
Axiowave Networks
100 Nickerson Road
Marlborough, MA 01752
EMail: jparker@axiowave.
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RFC 2973 IS-IS Mesh Groups October 2000
9. Full Copyright
Copyright (C) The Internet Society (2000). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Balay, et al. Informational [Page 8]
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