JUNOS Cookbook

Chapter 11. IS-IS

Introduction

The Intermediate System-to-Intermediate System ( IS-IS) protocol is an IGP that routes packets within a single autonomous system (AS), or domain. IS-IS is based on the DECNET Phase V network technology, which was developed at Digital Equipment Corporation (DEC) in the 1980s and was initially standardized by ANSI as the International Organization for Standardization (ISO) intradomain protocol in ISO/IEC 10589. The first version of IS-IS was designed to work on the OSI Connectionless Network Protocol (CNLP). RFC 1195, published in 1990, added extensions to support IP routes.

As an IGP, IS-IS works within a routing domain, which usually corresponds to an administrative boundary, and focuses on determining the most efficient routes to destinations within a domain. This is in contrast with EGPs, whose primary focus is on policy rather than on the most efficient routing. An IS-IS routing domain consists of end systems, which send and receive packets, and intermediate systems (the ISO term for a router), which receive and forward packets.

IS-IS is a link-state protocol and uses link-state protocol data units (link-state PDUs, or LSPs) to describe the network topology. Each IS-IS router generates LSPs that describe the topology, along with IP routes, checksums, and other information, and floods the LSPs throughout the domain. Each router ends up with a link-state database that describes the same network topology. Once the router has the complete network ...