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Integrated IS-IS Routing Protocol Concepts

Chapter Description

This chapter reviews the basic concepts underlying the design of the IS-IS routing protocol and also discusses the two-level hierarchy for controlling distribution of routing information within IS-IS areas and between them: Level 1 and Level 2, respectively.

Addressing Concepts in Integrated IS-IS

This section is a short prelude to Chapter 4, "Addressing in Integrated IS-IS," introducing only the key concepts for addressing in the IS-IS environment.

As a protocol originally designed for routing CLNP packets, IS-IS used the node-based addressing scheme of CLNP as its basic addressing premise. Integrated IS-IS, which can be used for routing IP packets, inherits many concepts from the original specification, including the CLNS addressing scheme for identifying network nodes. Therefore, it is a fundamental requirement that even when Integrated IS-IS is used in an IP-only environment, the nodes must have ISO NSAP addresses (referred to as NETs). However, IP requires the links to be addressed with IP subnets. Fortunately, the format for CLNS addresses used on IP routers is simple; most people can deal with the two addressing schemes and solve the single issue of routing IP.

Chapter 4 reviews CLNS addressing and provides practical examples of how they are used on IP routing. IP addressing on the links conforms to basic IP addressing principles and has no relationship with the CLNS addressing scheme. The latter exists solely for use by the IS-IS protocol.

As Integrated IS-IS for IP routing is enabled on the interfaces, the IP subnets are automatically added to the router's LSP by using the IP reachability and related TLVs introduced by RFC 1195. The IP prefixes are then assembled into an IP Link-State database, which is the fed to SPF algorithm to determine the best routes.