Hour 74: The state of IS-IS

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.

One comment

  1. The main reason when I am involved is that IS-IS requires extra licenses in the device operating system. For almost every Enterprise, the extra cost cannot be justified against the limited advantages. OSPF works quite well for the Enterprise.

    Service Providers use IS-IS to overcome the limitation Area 0 in OSPF. IS-IS is a much simpler protocol, with far fewer features and allows for multiple areas ( Level 1 and Level 2). This suits service provider networks which must be simple and large.


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