IS-IS on FRR - Is Anyone Running It?

Mark Tinka mark.tinka at
Mon Apr 6 09:36:27 UTC 2020

On 6/Apr/20 11:25, Saku Ytti wrote:

> Quite and then you specified but for ISIS you run 8000. I'm only
> talking about your ISIS here, not rest. And that 8k doesn't do
> anything useful,

Except for our CSR1000v on ESXi. We only ever needed it when we went
that route. We didn't need it prior to that.

>  so you can run 1500B to your FRR and not lose
> anything.

Which is my point - there is nowhere in FRR to set this, that is
documented or has worked from experiment/general knowledge.

> As I explained this is not true, by standard ISIS cannot do this for
>> 1500B MTU. As ISIS is 802.3 and 802.3 cannot send larger frames.
> Cisco has proprietary hack to do EthernetII for hellos, but even that
> doesn't guarantee anything, because ISIS will still come up, even if
> it receives subMTU hello as 802.3.

Again, this is only required on our CSR1000v ESXi deployments, because
our experience was that IS-IS would not establish because neighbors were
running at a different MTU from the RR's, and IS-IS on the RR's was
trying to setup adjacencies at a physically higher MTU than ESXi could

So lowering it fixed it, and we just picked 8,000. We could have picked
2,000, or 1,500, or whatever. The actual number doesn't matter, as long
as it's the same on the boxes that need it.

We spent about 6hrs troubleshooting this back in 2014, so we didn't put
it in just for fun.


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