What is IS-IS?

IS-IS (Intermediate System to Intermediate System) is a link-state interior routing protocol, similar to OSPF, widely used in large provider networks.

IS-IS was designed for the OSI protocol suite, where a router is called an "intermediate system", and was later extended to carry IP routes (RFC 1195). Like OSPF, every router floods descriptions of its links and neighbours (in IS-IS these are link-state PDUs), builds an identical database, and runs a shortest-path-first calculation using link metrics. The big structural difference is that IS-IS runs directly over layer 2, not inside IP packets, so it doesn't depend on IP addressing to form adjacencies and is harder to attack from a remote IP network. Each router is identified by a Network Entity Title (NET) rather than an IP router ID. Instead of OSPF's numbered areas around a backbone area 0, IS-IS uses two levels: Level 1 for routing within an area and Level 2 for routing between areas, with routers able to take part in both. Its information is carried in type-length-value fields, which made it easy to extend, for example for IPv6 and traffic engineering, within the same protocol instance.

Choosing between IS-IS and OSPF is often a matter of history and habit. Many large ISPs settled on IS-IS, while enterprises more often run OSPF. Both fill the same role as an IGP: carry loopbacks and infrastructure links so that BGP can reach its next hops. Newcomers usually find the NET format and the Level 1 and Level 2 terminology the hardest part; the concepts of adjacencies, flooding and SPF carry straight over. The OSPF Fundamentals lesson teaches link-state routing in a way that applies equally to IS-IS.