Re: IS-IS Flat Network Size

[Matthew Petach via NANOG <nanog () lists nanog org>]

On Tue, Jul 28, 2026 at 12:44 AM Saku Ytti <saku () ytti fi> wrote:

> This is absolutely the norm. People framing it as some engineering
> challenge that needs consideration are unnecessarily creating
> complexity and concern where there is none.
>
> Tier1s run global flat level2 at this scale, and have since forever.
>
> Outage information cannot propagate faster than light, no matter how
> you. bake it.
>
> The justification should be the opposite, you should have a strong
> reason not to run flat IGP, if you don't have it, run flat.
>
> You will struggle to justify any of what is proposed here, regarding
> SPF time, regarding convergence time, that these can be improved by
> adding complexity. SPF and convergence don't even matter in any modern
> design, because when you converge, you converge for all single
> failures too, that is, on link-down, you immediately forward around
> the problem, instead of waiting for SPF, since waiting for SPF takes a
> long time in any scale, even intraDC.

Hi Saku,

I apologize for not being clearer in my original message.  :(

I didn't mean to sound like I was concerned about L2 IS-IS scaling
or convergence at all.  You're absolutely right, you can run a global
flat L2 IS-IS network with thousands of nodes without concerns about
IGP routing table size or convergence times.

The point I was trying to make is that just because a routing protocol
supports doing that doesn't mean it's a good idea operationally.  There
are other issues that come into network design that often suggest a single
flat IGP might not give you enough knobs to allow you to control traffic
flows and impact radius from unusual externalities.

It was a long-winded and meandering way of saying "just because you
CAN, doesn't necessarily mean you SHOULD."   ^_^;

Thanks!

Matt
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