%OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet0/0 from EXSTART to DOWN, Neighbor Down: Too many retransmissions
%OSPF-5-ADJCHG: Process 1, Nbr on FastEthernet0/0 from DOWN to DOWN, Neighbor Down: Ignore timer expired

If error messages like the ones shown above appear in the syslog, the following command will help: "debug ip ospf events". This command can be used to verify the MTU value that is included in the OSPF calculation. The result could look like this, for example.

OSPF: Rcv DBD from on FastEthernet0/0 seq 0x12A6 opt 0×52 flag 0×7 len 32 mtu 1504 state EXSTART

In order to correct this error, there are these possibilities depending on whether it is a router or a switch or should take place directly in the routing process.

  • on a switch: system mtu 1500
  • on a router: ip ospf mtu-ignore
  • in the OSPF routing process: system mtu routing 1500