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IS-IS Flat Network Size


From: Tom via NANOG <nanog () lists nanog org>
Date: Mon, 27 Jul 2026 17:08:36 -0000

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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