OSPF MTU Mismatch Issue

 

This problem is common when running OSPF between a switch (i.e 3550 or 3560) and a router. The error message that is generated when this problem occurs is:

%OSPF-5-ADJCHG: Process 1, Nbr 150.8.5.5 on Vlan258 from DOWN to DOWN, Neighbor Down: Dead timer expired
%OSPF-5-ADJCHG:Process 1, Nbr 150.8.2.2 on Vlan258 from EXSTART to DOWN, Neighbor Down: Too many DBD retransmitions

Theresolution is to either adjust the system MTU on the switch or have OSPF ignore the MTU.

A)Change the system MTU on the switch (system mtu 1500 or system mtu routing 1500)

B)Have the router and/or switch ignore the MTU (ip ospf mtu-ignore)

C) Change the interface MTU on the router (GigE only)

Notethat using the system mtu 1500 option requires a reboot of the switch but the system mtu routing 1500 does not require a reboot.

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