The command “system mtu 1500” on switches is the default one. Even
when value is changed, it will not be stored in neither running-config
nor startup-config. On Catalyst 3550, this information is stored in a
separate file on the flash. On Catalyst 3560, you can’t see it, unless
you do “show system mtu”.
This is one of the well-know little gotchas on the actual lab exam.
You *have* to know how to solve this. Hence, when configuring routing
protocols on switches, make sure you know what their MTU is.
Part of the OSPF adjacency negotiation is MTU. This has to be the same on both ends of the link before the neighbor comes up
Then there are several ways to fix it. First is change the switch MTU (best practice) , second is change the MTU on the router’s interface (have seen this fail, not able to set manually by user) and third is use the ‘ip ospf mtu-ignore’ command on both interfaces.