RFC 2973 (rfc2973) - Page 2 of 8
IS-IS Mesh Groups
Alternative Format: Original Text Document
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
