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