Re: System MTU question

One important thing to note is that you might break stuff in the lab if you
were suppose to configure MTU for q-in q lab and later you changed System
MTU on your switch to fix OSPF issue.

IMHO,Best way to fix this in the lab would be ip ospf mtu-ignore under the
interface on your router as you cannot use this command on your switch .

On Sun, Oct 18, 2009 at 7:02 AM, Rick Mur <rmur_at_ipexpert.com> wrote:

> Indeed, if the switch wasn't configured for another MTU, but just left
> to thedefault of 1500. You don't need to specify any of the commands.
> Just keep it in mind when troubleshooting this.
>
> You can get the same issue with PPPoE config (virtual-template vs
> dialer MTU).
>
> --
>
> Regards,
>
> Rick Mur
> CCIE2 #21946 (R&S / Service Provider)
> Sr. Support Engineer IPexpert, Inc.
> URL: http://www.IPexpert.com
>
> On 18 okt 2009, at 13:52, Ryan West wrote:
>
> > Justin,
> >
> > Did you configure QinQ tunneling during your switching section? If
> > you didn't, it's most like left over from a previous lab, a wr er
> > does not reset system mtu. Overconfiguration is not going to cost
> > you points, only time and you need the MTU's to match for the
> > adjacency to form. It's not IOS version specific, I just wasn't
> > 100% of the syntax of the interface level command, I typically hit
> > the question mark there to make sure :)
> >
> > -ryan
> >
> > -----Original Message-----
> > From: Justin Mitchell [mailto:jgmitchell_at_gmail.com]
> > Sent: Sunday, October 18, 2009 7:49 AM
> > To: Nadeem Rafi
> > Cc: Ryan West; Cisco certification
> > Subject: Re: System MTU question
> >
> > Is it IOS version specific then? I'm reviewing the final configs for
> > the lab and I don't see either command being issued.
> >
> > Justin G. Mitchell
> > http://www.google.com/profiles/jgmitchell
> >
> >
> >
> > On Sun, Oct 18, 2009 at 6:45 AM, Nadeem Rafi <nrafia_at_gmail.com> wrote:
> >> yes Ryan... its correct. Its under interface.
> >>
> >> On Sun, Oct 18, 2009 at 2:38 PM, Ryan West <rwest_at_zyedge.com> wrote:
> >>>
> >>> 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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> >>>
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Received on Sun Oct 18 2009 - 14:01:24 ART