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.
IS-IS Route Leaking from Level 2 to Level 1
RBFS supports Level 2 to Level 1 route leaking in IS-IS. This feature allows Level 1 routers to learn specific Level 2 routes that are not automatically leaked to Level 1. By default, Level 1 routers reach other IS-IS levels (outside their area) by sending packets to the nearest Level 1/2 router via a default route.
In an IS-IS environment with LDP, Level 1 routers need to learn the loopback addresses of routers outside their area to establish LSPs (Label Switched Paths). Therefore, specific IP addresses must be leaked from Level 2 to Level 1 to provide required routing information. This feature allows a policy-based configuration on the Level 1/2 routers that connect the two levels. Route leaking selectively leaks particular Level 2 routes into Level 1 areas using predefined route policies.