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IS-IS Routing Protocol

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IS-IS: the ISP's link-state protocol

IS-IS (Intermediate System to Intermediate System, ISO 10589) is a link-state routing protocol like OSPF — routers exchange topology information and each runs Dijkstra's SPF algorithm independently. Unlike OSPF, IS-IS runs directly over Layer 2 (no IP header), making it slightly more robust during network instability.

Virtually every large ISP backbone runs IS-IS — Cisco, Juniper, Nokia, and Arista all treat it as a first-class IGP.

Packets in motion — click to inspect:

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