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

The resolution 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)

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

About Brian Dennis, CCIE #2210:

Brian Dennis has been in the networking industry for more than 22 years, with a focus on Cisco networking for the past 16 years. Brian achieved his first CCIE in Routing & Switching in 1996, and is currently the only ten year CCIE that holds five CCIE certifications. Prior to working with INE, Brian taught and developed CCIE preparation courses for various well known training organizations. Brian not only brings his years of teaching experience to the classroom, but also years of real world enterprise and service provider experience.

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4 Responses to “OSPF MTU Mismatch Issue”

  1. Vladimir says:

    Which method is preferred for production environment?
    Changing switch system mtu (any side effects?) or “ip ospf mtu-ignore”?

    (not taking into account that switch requires reboot to change system mtu, run into it once when IRB bridging on a router to a 3750 switch)

  2. daghighi says:

    I usually use solution B, but in some cases, it was enough to configure “ip ospf mtu-ignore” on the switch, and in some other cases, I had to configure “ip ospf mtu-ignore” on both the switch AND the router.

    I had not been able to identify why. Any hint ?


  3. In answer to the question about which device to enter the command ‘ip ospf mtu-ignore’ on, you need it on the one with the smaller MTU.

    Basically, the one with the smaller MTU is getting DBD (DataBase Descriptor) messages from the other device that indicate a bigger MTU than the local device is capable of handling. So the local device ignores the packets.



  4. Wassim says:

    I have two ADSL access links connected in a point-to-multipoint fashion to the service provider router. My routers were stuck at OSPF Exstart state.
    I read from Cisco that “ip ospf mtu-ignore” command was rare in its usage. But I put it and immediately my OSPF neighborship went DONE !


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