IS-IS in Service Provider Networks is still one of the most reliable choices for carrying loopbacks, infrastructure links, MPLS reachability, and Segment Routing information across large backbones. It is quiet, scalable, and protocol-flexible, but it also has a few operational details that can surprise engineers who mostly learned OSPF first.
This guide focuses on practical IS-IS design and operations for provider-style networks: levels, addressing, metrics, authentication, Segment Routing, and the troubleshooting checks that quickly separate a simple adjacency issue from a real control-plane problem.
