Re: System MTU question

ou can also use system mtu routing 1500 in your switch to fix OSPF issue as
this will be only used for routing but for Q-in-Q lab switch MTU will be
still used as 1504 ( if already set with system mtu command)

On Sun, Oct 18, 2009 at 2:01 PM, Ramanpreet Singh
<sikandar.raman_at_gmail.com>wrote:

> 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
>> >>>
>> >>>
>> >>> Blogs and organic groups at http://www.ccie.net
>> >>>
>> >>>
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>>
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Received on Sun Oct 18 2009 - 14:13:40 ART