The Art of Network Engineering What is IS-IS?
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Dec 17, 2025 Russ White, a seasoned network architect with expertise in routing protocols, and Mike Bushong, a practical network engineer, dive deep into the world of IS-IS. They explore why BGP dominates but often lacks flexibility, the architectural benefits of separating underlay and overlay networks, and the simplicity of IS-IS. The discussion highlights the declining deep protocol knowledge among engineers, the operational risks involved, and why IS-IS could be the underrated hero of modern data centers. Hands-on tips and resources for learning IS-IS also make an appearance!
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BGP Is A Policy System, Not A Fast IGP
- BGP is policy-rich and intentionally slow, so it doesn't suit fast-converging data center underlays.
- Russ White argues BGP acts like a policy distribution system rather than a pure fast IGP.
Separate Underlay And Overlay
- Separating underlay and overlay constrains failure and security domains.
- Russ prefers an ISIS underlay with a BGP overlay to avoid blending infrastructure and workload tables.
ISIS Is A Layer-2 Routing Protocol
- ISIS runs at Layer 2 and was designed for OSI CLNP, not IP, which avoids relying on IP for routing.
- That layer-2 design prevents multi-hop ISIS and reduces certain attack surfaces.