IS-IS on FRR - Is Anyone Running It?

Mon Apr 6 10:31:59 UTC 2020

On Mon, 6 Apr 2020 at 13:12, Mark Tinka <mark.tinka at seacom.mu> wrote:

> > https://github.com/FRRouting/frr/blob/58980443821edf95719984e01f31720bd1dc7f79/isisd/isis_constants.h#L172-L180
> >
> > But as long as you don't pad, your PDU shouldn't exceed 1500B.
>
> Good man, you gave me an idea.

There are other interesting implications about this high MTU commit -
https://github.com/FRRouting/frr/commit/316a98ecd151dfba86a4a87e11c98cb2a0e94518

If in FRR you generate large LSP (many interfaces or prefixes), FRR
may actually generate as large LSP as MTU allows. This means it
doesn't interoperate between Cisco and Juniper, which will fragment
generated LSPs at 1492B, no matter what CLSN MTU you have. On Cisco
you can increase LSP MTU to 4352, on Juniper you cannot increase above
1492B. So FRR should have an addition of LSP-MTU which should default
to 1492B to avoid interoperability issues when it must generate large
LSP PDU.

So better not make config where FRR needs to inject larger LSP PDU,
might be more excitement than what people would like. Someone can test
what happens when you redistribute more prefixes than can fit in 1492B
LSP PDU and if those LSPs propagate to Ciscos and Juniper, blackhole
or crash the network.

-- 
  ++ytti