RE: System MTU question

Justin,

That's normal, probably a left over system mtu from QinQ configuration. You do have the option of setting of system mtu routing 1500 as a workaround for the 3560 and above series. You can also tell the OSPF process to ignore the MTU on a per-interface basis. Ip ospf mtu-ignore (I think).

-ryan

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of Justin Mitchell
Sent: Sunday, October 18, 2009 7:35 AM
To: Cisco certification
Subject: System MTU question

I am working on an OSPF configuration and watching the console
messages long enough I get to see one neighbor go up and down with the
message:

R3#
*Nov 7 21:13:10.955: %OSPF-5-ADJCHG: Process 1, Nbr 112.12.108.1 on
FastEthernet0/0 from EXSTART to DOWN, Neighbor Down: Too many
retransmissions
R3#
*Nov 7 21:14:10.955: %OSPF-5-ADJCHG: Process 1, Nbr 112.12.108.1 on
FastEthernet0/0 from DOWN to DOWN, Neighbor Down: Ignore timer expired

So I issue "debug ip ospf ev" and get the notice the following:

*Nov 7 21:11:12.815: OSPF: Rcv DBD from 112.12.108.1 on
FastEthernet0/0 seq 0x12A6 opt 0x52 flag 0x7 len 32 mtu 1504 state
EXSTART
*Nov 7 21:11:12.815: OSPF: Nbr 112.12.108.1 has larger interface MTU

I changed to the switch causing the problem and set "system mtu 1500"
reloaded and then the adjacency forms without any problems.

Is this normal? I usually don't set the MTU unless required for
overhead reasons.

Justin G. Mitchell
http://www.google.com/profiles/jgmitchell

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Received on Sun Oct 18 2009 - 07:38:58 ART