Chapter 4. IS-IS

Intermediate System-to-Intermediate System (IS-IS) is a link state routing protocol that is part of the OSI family of protocols. Like OSPF, it uses Dijkstra’s SPF algorithm to choose routes. IS-IS is a classless interior gateway protocol that uses router resources efficiently and scales to large networks, such as large Internet service providers (ISP).

Table 4-1 lists some IS-IS terms, acronyms, and their meanings.

Table 4-1. IS-IS Acronyms




Circuit ID

Identifies a physical interface on the router.

Complete Sequence Number PDU


A summary of a router’s complete LSDB.

Connectionless Network Protocol


OSI protocol used to provide the connectionless services.

Connectionless Network Services


OSI data delivery service ...

Get CCNP BSCI Quick Reference Sheets: Exam 642-901 now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.