IS-IS vs OSPF: Why Every Major Service Provider Chose IS-IS (and How It Powers Segment Routing)

IS-IS (Intermediate System to Intermediate System) is the dominant interior gateway protocol in service provider networks worldwide. If you work in or around SP infrastructure, IS-IS isn't optional — it's the foundation everything else (MPLS, Segment Routing, traffic engineering) runs on top of. This post breaks down why SPs chose IS-IS over OSPF, how its addressing and multi-level design work, and how it integrates natively with Segment Routing — the technology replacing RSVP-TE in modern backbones. Why Did Service Providers Choose IS-IS Over OSPF? This is the question OSPF-trained enterprise engineers always ask. The answer goes deeper than "it's what SPs use." 1. Protocol Independence (CLNS, Not IP) OSPF runs on top of IP — it uses IP protocol 89 and depends on IP addressing to function. IS-IS runs on CLNS (Connectionless-Mode Network Service) at Layer 2, next to IP rather than on top of it. Why this matters: IS-IS can carry any protocol's routing information through TLVs — IPv4, IP