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
