Re: IS-IS Flat Network Size

[Mikael Abrahamsson via NANOG <nanog () lists nanog org>]

On Mon, 27 Jul 2026, Tom via NANOG wrote:

> 1) Is anyone running a flat IS-IS L2 network with approx 300 routers? If not, what is the higher end you've seen?

As others have stated, 300 routers is (at least) 1-2 orders of magnitude 
lower than "this could be a problem" with modern hardware.

What I'd start to consider at (much) higher numbers than yours is number 
of routes you have in IS-IS, considering FIB update speed is in the order 
of tens of thousands per second on most hardware assisted platforms.

This means that if you have tens of thousands of routes, you want to make 
sure the platform you're using updates the important nexthops first (for 
instance loopbacks) in the FIB, otherwise your covergence time suffers.

--
Mikael Abrahamsson    email: swmike () swm pp se
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