Re: IS-IS Flat Network Size

[Dan Snyder via NANOG <nanog () lists nanog org>]

It really depends on what hardware you are using, how large of a fault
domain you want if there is an issue in the L2 area, and how quickly you
expect it to converge if there is a failure.

On Mon, Jul 27, 2026 at 2:55 PM Tom Beecher via NANOG <nanog () lists nanog org>
wrote:

> 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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