IS-IS for CCIE Service Provider: Why SPs Choose It Over OSPF and How to Master It in 2026
IS-IS (Intermediate System to Intermediate System) is the dominant interior gateway protocol in service provider networks worldwide, and it’s the primary IGP tested on the CCIE Service Provider v5.0 blueprint. If you’re studying for CCIE SP or working in an SP environment, IS-IS isn’t optional — it’s the foundation everything else (MPLS, Segment Routing, traffic engineering) runs on top of. Key Takeaway: Service providers chose IS-IS over OSPF decades ago for its TLV extensibility, protocol independence, and simpler flooding mechanics — and that decision has been validated repeatedly, most recently by IS-IS’s seamless integration with Segment Routing without requiring a protocol version change. ...