Did you know that OSPF neighbors do not move to the  FULL state with mismatched MTU?

I found this out at the weekend when I was working on some Data Centre switches, within the fabric these switches use jumbo MTU. So when I tried to peer a device that was not part of the fabric, I got stuck in EXSTART.

At first I was wondering if I had the OSPF configuration. I checked and double checked but all looked good.

Solved!

run “debug ip ospf adj” you will get a message similar to this.

*Nov 16 21:30:45.551: OSPF-1 ADJ Gi0/0: Nbr 10.1.1.1 has smaller interface MTU

Answer: match the MTU on both sides.
I had a read through TCP/IP Volume 1, It doesn’t mention MTU size anywhere. RFC 2328 does mention it.

If the Interface MTU field in the Database Description packet
indicates an IP datagram size that is larger than the router can
accept on the receiving interface without fragmentation, the
Database Description packet is rejected.

This is where wireshark comes in handy, I had to see this for myself.

The MTU isnt sent in the Hello packet its sent in the type 2 DBD packet, this is after the neighbors acknowledge each other (2WAY). See below.

mtu-ospf

Interesting

RH