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).