h1

IP OSPF mtu-ignore alternative

April 6, 2009

I came across the a command I think would make a great CCIE lab question.

Assume you busy with the lab, and previously a task in the switching section required you to do a dot1q tunnel where you had to change the SYSTEM-MTU on SW1 to 1504.  No beegy.
But you now at the OSPF section, where you have to setup ospf between R1 and SW1, BUT with the following restriction:
(you are not allowed to use the mtu-ignore command)

The usual fix on R1’s interface is prohibited
#interface Fa0/0
#ip ospf mtu-ignore

Hmmm, now what? R1 wont form an adjacency with SW1, due to a MTU mismatch. We obviously cant change the SYSTEM-MTU on SW1, cause that would break a previous question.

Typical behaviour when you have a OSPF MTU mismatch, is a neighbor finite state getting to EXSTART, retrying and eventually giving up.
We can see this on R1 if we do a “debug ip ospf adj”

1ospf-debug-12


Its is always easy to fix a problem if you know the right commands, and in this case it is the following on SW1:
#system mtu routing 1500

That will enable SW1 to still use a SYSTEM-MTU of 1504 as needed, but for routed traffic (ie the ospf stuff) the switch will use a MTU of 1500. Problem solved:

1ospf-debug21

Couple thing to know about this command:
– It is not interface specific like ‘ip ospf mtu-ignore’, it is applied globally on the switch.
– The routing MTU cannot exceed the SYSTEM-MTU.
– It is not available on the 3550’s
– Once the SYSTEM-MTU is active, changing the routing mtu does not require a reload.

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One comment

  1. Hi Ruhann, congrats on the digits! Will you post a review of the mobile lab?



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