Overview of IS-IS

Definition

Intermediate System to Intermediate System (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) extends and modifies IS-IS in relevant standards, which enables IS-IS to be applied to both TCP/IP and Open System Interconnection (OSI) environments. This type of IS-IS is called Integrated IS-IS or Dual IS-IS.

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

If IS-IS IPv4 and IS-IS IPv6 implement a feature in the same way, details are not provided in this chapter.

Purpose

As an Interior Gateway Protocol (IGP), IS-IS is used in Autonomous Systems (ASs). IS-IS is a link state protocol, and it uses the Shortest Path First (SPF) algorithm to calculate routes.