Re: System MTU question

Part of the OSPF adjacency negotiation is MTU. This has to be the same
on both ends of the link before the neighbor comes up. For a router
this is easy, as you can change it under the interface, but on a
switch you can only change it for the whole system. You usually see
this on rack rentals, as people practice with Q-in-Q (where you need
that extra MTU to add another 802.1Q header). It's always a good way
to troubleshoot a switch when an OSPF neighbor is not coming up, issue
the 'show system mtu' and most times you'll see 1508.

Then there are several ways to fix it. First is change the switch MTU
(which is the best practice in my oppinion), second is change the MTU
on the router's interface and third is use the 'ip ospf mtu-ignore'
command on both interfaces.

--
Regards,
Rick Mur
CCIE2 #21946 (R&S / Service Provider)
Sr. Support Engineer  IPexpert, Inc.
URL: http://www.IPexpert.com
On 18 okt 2009, at 13:34, Justin Mitchell wrote:
> 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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