OSPF adjacencies and MTU

As you all know if you are configuring OSPF an adjacency will not form with a neighbour router if the MTU is different on the connecting interface of each device. I ran into this issue recently when I was configuring a neighbour ship between a Cisco 877w and a Cisco 3750 switch. The thing that threw me off when I was troubleshooting why the adjacency would not form was that I could not find any indication of what the MTU was set to for the interfaces on either device. I checked and double checked but I could not get the adjacency to form so I enabled the debug ip ospf adj, here I could see the issue on the switch

error: OSPF: Nbr has smaller interface MTU

According to this error the MTU on the switch is bigger than the what is configured on the router (by default) so effectively the devices cannot commnuicate. No big deal right we will just change the MTU for the SVI on the switch however when trying to do this we got the following: % ip mtu is not supported on this interface

After some digging around on the internet I found that there were 3 ways to change the MTU on the c3750:

  • change the system MTU (system mtu 1500)
  • change the system MTU routing (system mtu routing 1500)
  • tell OSPF to ignore the MTU missmatch on the interface (ip ospf mtu-ignore)

Change the system mtu will require a reboot of the device, which is not such a hot idea in a production environment. Telling OSPF to ignore the MTU could cause other problems in communication so everything I found said to use this command with care. Changing the system mtu routing was the easiest and it seemed to have the least impact as it is a global command and does not require a reboot of the device.

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