I asked on comp.dcom.sys.cisco a few days ago about the advantages of IS-IS over OSPF. Dave Katz replied that IS-IS has proven to be very extensible while still allowing for backward compatibility. IS-IS was also overhauled around the same time that a lot of ISPs were building their networks. My questions are these. When is a good time to use IS-IS? Should an ISP that wants to build a scalable network go with IS-IS right now? Or is there a certain point where IS-IS becomes desirable over OSPF? Is there ever such a point? I would think it is better to pick an IGP and stick with it. Thanks for any comments. The real truth is that for a very long time cisco's OSPF sucked rocks. Big rocks. Boulders. Mountains. Entire asteroids. So when certain backbones asked about new IGP's, I pointed them at IS-IS. It wasn't mature at that point, but its implementation was head and shoulders above OSPF. Now, a couple of years later, both the IS-IS and OSPF implementations are much more stable. Yes, you want to make a careful decision here because having to change IGPs pretty much causes you a flag day. ;-( Important points to ponder: - OSPF has a very strong notion of the backbone area. IS-IS has a slightly different model with two levels. If your topology means that you'd have to run all routers as part of the OSPF backbone area, you're not going to be scalable. Similarly, if you would have to put everything into one IS-IS area, you're also in trouble. Check out the topology that you can see in your crystal ball. - IS-IS is more extensible. It already supports two networks layers and rumor has it that there's code for another forthcoming. Gotta go to OSPFv3 for this level of extensibility. - OSPF has a better metric, with enough dynamic range so that you can more easily achieve "optimal" routing. - There are more folks running OSPF, so the number of run-hours will grow faster and the code should be more stable. - The folks using IS-IS have some of the biggest, most stressed routers in the world. So I would expect cisco's IS-IS implementation to be more stressed in the scalability dimension. So what's best? It depends... Tony