Simple definition
IS-IS is a link-state routing protocol used especially in service provider networks to calculate paths within a routing domain.
Purpose and use
IS-IS, Intermediate System to Intermediate System, is a link-state protocol used to build routes within a routing domain.
How does it work?
Routers distribute topology information and calculate paths. Level 1 and Level 2 organize local-area and inter-area exchange.
Example and verification
When an adjacency is missing, compare neighbors' levels, circuit types, authentication and system identifiers.
Points to watch
IS-IS operates directly over the link layer and has no TCP or UDP port. IP connectivity between routers therefore does not prove that IS-IS works.
In short
Routes depend on consistent topology information and compatible adjacencies, rather than merely assigning interface IP addresses.