RFC 3719 (rfc3719) - Page 2 of 15
Recommendations for Interoperable Networks using Intermediate System to Intermediate System (IS-IS)
Alternative Format: Original Text Document
RFC 3719 Interoperable Networks using IS-IS February 2004
1. Introduction
In theory, there is no difference between theory and practice.
But in practice, there is.
Jan L.A. van de Snepscheut
Interior Gateway Protocols such as IS-IS are designed to provide
timely information about the best routes in a routing domain. The
original design of IS-IS, as described in ISO 10589 [1] has proved to
be quite durable. However, a number of original design choices have
been modified. This document addresses differences between the
protocol described in ISO 10589 and the protocol that can be observed
on the wire today. A companion document discusses differences
between the protocol described in RFC 1195 [2] for routing IP traffic
and current practice.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY" in
this document are to be interpreted as described in RFC 2119 [3].
2. Constants That Are Variable
Some parameters that were defined as constant in ISO 10589 are
modified in practice. These include the following
(1) MaxAge - the lifetime of a Link State PDU (LSP)
(2) ISISHoldingMultiplier - a parameter used to describe the
generation of hello packets
(3) ReceiveLSPBufferSize - discussed in a later section
2.1. MaxAge
Each LSP contains a RemainingLifetime field which is initially set to
the MaxAge value on the generating IS. The value stored in this
field is decremented to mark the passage of time and the number of
times it has been forwarded. When the value of a foreign LSP becomes
0, an IS initiates a purging process which will flush the LSP from
the network. This ensures that corrupted or otherwise invalid LSPs
do not remain in the network indefinitely. The rate at which LSPs
are regenerated by the originating IS is determined by the value of
maximumLSPGenerationInterval.
Parker Informational