Jumbo Frames - OSPF

Unanswered Question
Dec 17th, 2007

Hi !

We are trying to implement Jumbo Frames on one VLAN.

As soon as I enter the MTU command on the VLAN interface - OSPF re-synchronizes and loads correctly...

Dec 17 14:08:03.844 CET: %OSPF-5-ADJCHG: Process 205, Nbr xxx.xxx.xxx.xxx on Vlan102 from LOADING to FULL, Loading Done

BUT redistributed routes from the other euqipement is not sent to me anymore.

As soon as I remove the MTU command - the redistr. route is there again.

Could you please help me ?

Is there any additional command on OSPF I have to use ?

br

Hans

I have this problem too.
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Kevin Dorrell Mon, 12/17/2007 - 05:34

Generally, OSPF will not properly form an adjacency between two interfaces with different MTU. (OTOH, I am a bit surprised that one side says it is FULL .. but I don't remember the exact symptoms. Maybe it forms the adjacency but does not pass the database.)

There are two ways you can tackle this: one is to ensure you have the same MTU on both sides, and the other is some command like ip ospf mtu-ignore on the interface in question.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cr/hirp_r/rte_osph.htm#wp1000796

Kevin Dorrell

Luxembourg

paul.matthews Mon, 12/17/2007 - 08:01

Kevin is right on the adjacency formed but not the full exchange, though what you have looks a little odd.

Make sure both/all routers on the VLAN are configured with the same MTU. That should be the case anyway - any other configuration could lead to packet loss.

When getting to database exchange phase, one router becomes master and the other slave. If the master is configured with a smaller MTU, you may find things work, if the slave is smaller and you have a reasonable database it will fail - OSPF will reject received frames if they are above the configured MTU.

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