[c-nsp] is-is question

Aaron dudepron at gmail.com
Thu May 14 18:47:35 EDT 2009

There are several Tier 1 isps that just run Level-1 only.


On Thu, May 14, 2009 at 16:56, Mark Tinka <mtinka at globaltransit.net> wrote:

> 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.
> Cheers,
> Mark.
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