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. _______________________________________________ NANOG mailing list
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