Re: IS-IS Flat Network Size

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

On Thu, 30 Jul 2026 at 09:46, Matthew Petach <mpetach () netflight com> wrote:

> 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."   ^_^;

I'll see your long-winded and meandering and raise you this:

Of course I agree with the general principle here, but in this case,
just because you can, you should. Because NOT doing flat IGP is far
more complex, and adding complexity requires justification.

And as we know, all of these devices are extremely fragile and very
low quality software, we run into customer-affecting bugs all the time
and it's entirely impossible to have any strong confidence that what
you're delivering works well. So you really need to pick your battles,
which problems most. benefit from careful thought, which problems do
not need it.

I'm fairly confident (happy to be proven wrong, if provided access)
that in any network configuration I review, I'll find catastrophic
mistakes in production configuration, in control-plane protection, in
QoS, and highly suboptimal configurations in regards to convergence
and redundancy.  And these will not be complex or esoteric, but fairly
common, simple mistakes. No one seems to have time to even get basics
rights, doing deep dive on issue that is known to be very barren
ground for issues is not justifiable.


-- 
  ++ytti
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