Re: IS-IS Flat Network Size

[Saku Ytti via NANOG <nanog () lists nanog org>]

I've run an ISIS network of a few thousands nodes, back when the
network included c2k series routers in addition to things like GSR
(i.e. a couple decades ago), so very slow control-planes.

According to Abe Martey's IS-IS network designs solutions book you can
approximate SPF complexity in nonhighly meshed networks as O(LlogN)
(links, nodes). When we combined two large ISIS networks, and I was
worried about the computational cost on the lower end devices, I used
this to approximate what the SPF run time would be after migration,
and it was spot on.

In some ISIS standard bodies public mailing lists some Chinese
networks have spoken about many orders of magnitudes larger flat
level2 ISIS topologies.

So I wouldn't worry about it, 300 was nothing 20 years ago, and it's
nothing today.

On Mon, 27 Jul 2026 at 20:48, Tom via NANOG <nanog () lists nanog org> 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?
>
>  Thanks.
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-- 
  ++ytti
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