IGP choice

Mark Tinka mark.tinka at seacom.mu
Fri Oct 30 16:43:39 UTC 2015

On 30/Oct/15 15:34, Matthew Petach wrote:

> It is rather nice that IS-IS does not require level-2 to be
> contiguous, unlike area 0 in OSPF.  It is a valid topology
> in IS-IS to have different level-2 areas connected by
> level-1 areas, though you do have to be somewhat
> careful about what routes you propagate into-and-back-out-of
> the intervening level-1 area.

I found Route Leaking in IS-IS to be a moot endeavour because if one
wants to keep absolute routing inside the IGP, you'll want to have the
core and Loopback interface addresses in the IGP, particularly if you're
running an MPLS network.

In such a case, the only real gain you get from multi-level IS-IS is a
little quietness re: the LSP's being propagated within a particular
Level-1 Area. However, things like PRC (Partial Route Calculation) and
iSPF (Incremental SPF) help a lot here when you have a flat Level-2
IS-IS domain.


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