IS-IS on FRR - Is Anyone Running It?

Mon Apr 6 07:58:37 UTC 2020

On Mon, Apr 6, 2020, at 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.

I won't speak for Mark, but NO, when you're carrying somebody's else's traffic you do your best to have the MTU on each and every backbone link "high enough" : preferably in the 9200(bytes) range, so you can easily transport 9000(bytes) client packets, and by no means so small that you need to fragment 1500(IP)/1514(Eth) byte packets. If things are really-really bad, 1600 bytes towards the edge.

> 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.

Ethernet cannot signal MTU. But if you have equipment at both sides of a P2P link, you don't need any signalling. You check the MTU suits your needs and put it in statically. Same for NNIs : the MTU is signalled in a document called "contract" or "agreement". And no, the guy that is being woken up at 3am for an issue, if he/she doesn't know that we run Jumbo, then he/she should start updating the CV.

Back to the original question, I would expect FRR to be able to manually specify a MTU/frame-size, like any other decent NOS (even if it's not a full NOS).