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

IS-IS dominates service provider networks due to its CLNS-based protocol independence, TLV extensibility that natively absorbs new features like Segment Routing without protocol redesign, and simpler two-level flooding compared to OSPF's complex multi-type LSA scoping. The article explains NET/NSAP addressing, IS-IS's multi-level hierarchy, and why these architectural advantages made IS-IS the universal SP choice over OSPF.