Comments on: OSPF MTU Mismatch Issue

By: Wassim

Wassim — Tue, 15 Jun 2010 09:26:17 +0000

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 !
Thanks

By: Andrew Mulheirn

Andrew Mulheirn — Mon, 12 Jan 2009 19:17:40 +0000

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.

HTH

Andrew

By: daghighi

daghighi — Wed, 02 Apr 2008 18:59:17 +0000

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 ?

Thanks,
Navid

By: Vladimir

Vladimir — Wed, 06 Feb 2008 00:44:31 +0000

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)