As many of you might know, IS-IS is a link-state interior routing protocol that uses, like OSPF, the Dijkstra algorithm for computing the best path in a network. I’ve worked with quite a few medium and large enterprise networks where OSPF was the main IGP but I have never seen IS-IS implemented in any of these. For this reason, a question came to my mind; how widespread is IS-IS? Since IS-IS isn’t in the blueprint for CCIE R&S, I thought that it was probably used only in legacy systems or for small companies, that for whatever reason, decided to implement IS-IS instead of OSPF. Boy was I wrong! In fact, after asking the opinion to some other experienced network engineers they told me that most Service Providers use IS-IS as their primary IGP. You can find their responses in this thread: http://www.techexams.net/forums/ccie/94074-state.html Now I could discuss all day about the pros and cons for OSPF vs IS-IS but instead, I invite you to watch this podcast that a fellow TE member pointed out, where CCIE’s debate about this: http://packetpushers.net/show-89-ospf-vs-is-is-smackdown-where-you-can-watch-their-eyes-reload/.
I think one of the main reasons I haven’t seen IS-IS in any of the networks I’ve worked with was because they weren’t Service Providers, large enough or had DMPVN ‘s (which are not supported with IS-IS). For the moment, I won’t study in detail IS-IS as it is not, like I said earlier, in the CCIE R&S blueprint.