Overview of IS-IS

Definition

Intermediate System-to-Intermediate System (IS-IS) is an Interior Gateway Protocol (IGP) that runs within an autonomous system (AS). IS-IS is also a link-state routing protocol, using the shortest path first (SPF) algorithm to calculate routes.

Purpose

IS-IS is a dynamic routing protocol initially designed by the International Organization for Standardization (ISO) for its Connectionless Network Protocol (CLNP).

To support IP routing, the Internet Engineering Task Force (IETF) extended and modified IS-IS in RFC 1195. This modification enables IS-IS to apply to TCP/IP and OSI environments. This type of IS-IS is called Integrated IS-IS or Dual IS-IS.

IS-IS stated in this document refers to Integrated IS-IS, unless otherwise stated.

In addition to IPv4 networks, IS-IS also applies to IPv6 networks to provide accurate routing information for IPv6 packets. IS-IS has good scalability, supports IPv6 network layer protocols, and is capable of discovering, generating, and forwarding IPv6 routes.