IS-IS on FRR - Is Anyone Running It?

Mark Tinka mark.tinka at
Mon Apr 6 08:50:28 UTC 2020

On 6/Apr/20 07:51, Saku Ytti wrote:

> I'm not sure what 'globally our IS-IS domain runs 8000 bytes' means.
> Your LSP MTU is like 1492B, there isn't a mechanism to fragment and
> reassemble LSP in-transit. ISIS network doesn't support different MTU
> sizes and I've not heard anyone being brave enough to increase LSP MTU
> above 1492B.
> The only thing that is larger in your network is hellos, and I'm not
> even sure how that works, considering 802.3 cannot signal larger
> frames than 1500B.

I mean that every router and switch interface in our network runs at
9,178 bytes. Ordinarily, we wouldn't need to do anything to IS-IS as it
would just take that and run with it. However, VMware ESXi does not
support anything larger than 9,000 bytes, and that is where we run our RR's.

So to make it all harmonious, while the actual data plane in our routers
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.

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 information about the NANOG mailing list