IS-IS on FRR - Is Anyone Running It?

Mon Apr 6 09:05:21 UTC 2020

On Mon, 6 Apr 2020 at 11:50, Mark Tinka <mark.tinka at seacom.mu> wrote:

> and switches can forward up to 9,178 bytes, we set IS-IS MTU to 8,000
> bytes on all of them, as a lowest common denominator so that our RR's
> can participate in the IS-IS domain.

And the only thing this 'ISIS MTU' (think you mean CLNS MTU) does, is
for some cases make ISIS hellos larger. If you don't pad ISIS hellos,
or if you have standard compliant ISIS, it doesn't do anything past
1500B.

> It's got nothing to do with trying to transport CLNS frames or fragment
> them, since as you rightly point out, fragmentation is not supported at
> Layer 2.

More accurately ISIS doesn't have a way to fragment LSP into two LSP,
so the whole ISIS network needs the same LSP-MTU, which has nothing to
do with 'clns mtu', LSP-MTU does not increase with increasing CLNS
MTU. There is no utility of having a harmonised CLNS MTU, so if you
want to speak to FRR, you can just do 1500B and it's fine.

-- 
  ++ytti