[c-nsp] is-is question

Mark Tinka mtinka at globaltransit.net
Thu May 14 16:56:10 EDT 2009

On Wednesday 14 May 2008 03:31:00 am victor wrote:

> Because of a recent change of the organizational
> structure of the company I'm employed by I was given an
> order to migrate all the current routing infrastructure
> (a couple of c7604, c7201 and a dozen of c4924) from OSPF
> to is-is. I've never worked with is-is before and after a
> bit of studying I feel comfortable enough with the
> concept and a possible migration strategy. The only
> question I have so far is what is-is level should I
> prefer? With OSPF all devices reside in Area 0. Naturally
> the closest match from is-is world would be to configure
> only one level-1 area. But during my search the web for
> the best practices I saw somewhere that with the same
> result I could put each device into  separate areas
> configuring only level-2 interarea routing and completely
> abandon idea of level-1. I'd very much like to hear your
> opinion on this matter.

For a direct comparison, what Steinar mentioned would equate 
to OSPF's Area 0, i.e., a single L2 level. We use multiple 
levels, L1 within the PoP, L2 between the PoP's, but our 
design may be a little more complex than what you need.

Given the integration between prefix and topology 
information in OSPFv2, a single L2 level in IS-IS would 
scale slightly better than a single Area 0 in OSPF, because 
IS-IS separates topology from prefix information, making 
partial SPF runs more efficient.

However, this isn't an issue with OSPFv3 anymore - it's been 
addressed for OSPF.

Be careful if you're running (or plan to run) IPv6. I'd 
recommend configuring multi-topologies for IS-IS, as life 
will be a lot easier once you start turning on IPv6.



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