Re: IS-IS Flat Network Size

[Tom Beecher via NANOG <nanog () lists nanog org>]

Yeah 300 nodes in a single flat L2 area generally shouldn't be a problem.

You may need to tweak SPF delay depending on geographic distribution and/or
total size of the LSDB , but it should be very doable. Realistically if the
LSDB size is a thing you probably shouldn't be running a flat area anyways,
but with a little tweaking you can get away with it just fine.

On Mon, Jul 27, 2026 at 2:41 PM Mark Tinka via NANOG <nanog () lists nanog org>
wrote:

> On 27/07/2026 19:08, Tom via NANOG wrote:
>
> > Hello,
> >
> > I'm wondering what some practical/real experiences have been with
> building out a large flat IS-IS network. I've read a lot of threads on
> conjecture and older docs on what the vendors recommend but haven't found
> anything yet or real world examples. Two questions,
> > 1) Is anyone running a flat IS-IS L2 network with approx 300 routers? If
> not, what is the higher end you've seen?
> > 2) On aggregation routers, what is the upper end of IS-IS adj to
> downstream routers? Would agg routers with approx 100 connections be an
> issue moving to IS-IS with the addition of keeping track of that many adj?
>
> With today's control planes, 300 nodes is not even a corn flake :-).
>
> We did that number back in 2009, on Cisco CRS-1's, ME3600X's and Juniper
> M320's, T320's and MX480's.
>
> The state-of-the-art has moved on a great deal, since then.
>
> Mark.
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