IS-IS Overview

The Intermediate System to Intermediate System (IS-IS) protocol is an interior gateway protocol (IGP) that uses link-state information to make routing decisions.

IS-IS is a link-state IGP that uses the shortest path first (SPF) algorithm to determine routes. IS-IS evaluates the topology changes and determines whether to perform a full SPF recalculation or a partial route calculation (PRC). This protocol originally was developed for routing International Organization for Standardization (ISO) Connectionless Network Protocol (CLNP) packets.

Because IS-IS uses ISO addresses, the configuration of the IPv6 and IPv4 implementations of IS-IS is identical.

This chapter discusses the following topics that provide background information about IS-IS:

• IS-IS Standards
• IS-IS Terminology
• ISO Network Addresses
• IS-IS Packets
• IS-IS Extensions to Support Traffic Engineering
• IS-IS Extensions to Support Route Tagging