Re: IS-IS Flat Network Size

[Mark Tinka via NANOG <nanog () lists nanog org>]

On 02/08/2026 13:21, Saku Ytti wrote:

> I think 'end-to-end' here can be defined in many ways. But I would
> consider seamless MPLS as end-to-end, and of course in this case you
> have no global IGP, you have metros having their own level2, and core
> having its own level2, none of them communicate.
> I've done that as well, and that's generally the approach if you
> cannot justify global L2, there is no real case for L1, L1L2, L2
> design.

Yes, large networks with Metro-E rings can either have a flat L2 domain 
that communicates between the core and metro, or can have BGP-LU 
separating them (as you say, the Seamless MPLS definition).

I suppose in either case, a single L2 domain remains as the only scope, 
whether separated by BGP-LU or not.

I have ran both topologies without issue, but admittedly, different scales.

Mark.
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