CCNP (BSCI) study question, Specifically IS-IS

Unanswered Question

Hi All

First I would like to wish you all Seasons Greetings. Hope one and all had a great Christmas and best wishes for 2008 and beyond.

To my question.

I am just going over my BSCI notes in preparation for taking the exam in the near future (I hope) and I have a question about how IS-IS would be used in the real world.

In my study material it is stated that L3 OSI routing is not implemented on Cisco routers as Cisco routers do not support IDRP. However the material states that IS-IS is used by ISP's and is still in use today as it is a robust and stable routing protocol.

This being the case, am I to understand that a single ISP would enclose their entire network in one "Domain" which consists of different "Areas" and would use L2 OSI routing between areas (given that they were using IS-IS of course), but would use BGP as the routing protocol to route traffic out of their network since Cisco does not support L3 OSI (IDRP) routing?

Best Regards & Thanks for any responses,


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Hi All

Ok, I think I was getting myself in a muddle.

As IS-IS is an IGP it is not designed for routing between different AS'es, so I guess BGP is the logical solution.

But then why would a single ISP use multiple "Domains" in their IS-IS network?

Anybody knowledgeable people or anybody doing CCIE SP who could shed some light on this for me please? It's just for my own knowledge and would be much appreciated.

Best Regards,



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