IS-IS Overview

IS-IS, or Intermediate System to Intermediate System, is an open standard routing protocol. ISO published the standard as a way to route datagrams as part of their OSI stack. IETF later republished the standard, and added IP route support.

It is a link-state routing protocol, similar to OSPF. It forms neighbor adjacencies, has areas, exchanges link-state packets, builds a link-state database and runs the Dijkstra SPF algorithm to find the best path to each destination, which is installed in the routing table.

IS-IS Segment Routing

IS-IS in RBFS supports segment routing based on RFC 8667. IS-IS Segment Routing enhances the IS-IS protocol by introducing new TLVs to carry segment routing information within IS-IS protocol packets. It introduces Segment IDs (SIDs) to represent different types of segments, such as prefix SIDs (node SIDs and anycast SIDs) and adjacency SIDs.

RFC and draft compliance are partial except as specified.

IS-IS Support Over Unnumbered Interfaces

An unnumbered interface is a point-to-point interface that is not explicitly configured with a dedicated IP address; instead, it borrows an IP address from a loopback interface. IS-IS can be set up over an unnumbered interface, enabling each router to use a loopback IP address to establish an IS-IS adjacency.

For details on configuring an unnumbered interface, refer to the Logical Interface Configuration section of the Interfaces User Guide.

Supported Platforms

Not all features are necessarily supported on each hardware platform. Refer to the Platform Guide for the features and the sub-features that are or are not supported by each platform.