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I've read that IS-IS is more popular than OSPF in Internet Service Providers due to historical reasons (mainly that initially IS-IS had better hardware support and wasn't for IP only).

But why did OSPF become more prevalent in Enterprise networks? The only possible explanation I've come up with is that OSPF is an open standard, while the IS-IS standard is not free.

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While IS-IS and OSPF work very similarly on the routing level (link-state, Dijkstra), likely the most important difference is that OSPF sits on top of IP (L3) while IS-IS sits on top of the data link layer (L2).

Accordingly, IS-IS requires L2 connectivity between peers which isn't always available (e.g. when L3 tunneling is used with VPN or similar). Therefore, OSPF is a more universal protocol and more suitable for being used throughout an enterprise network.

On the other hand, IS-IS is more universal in what kind of protocols it supports (it actually forms the base for Shortest Path Bridging 802.1aq) - but since other L3 protocols have grown out of use, OSPF v2/v3 are pretty much all you need.

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