From: Matt Mullen (mullenm@gmail.com)
Date: Tue Apr 12 2005 - 08:36:40 GMT-3
Ok, that makes sense. So even if the switch was rebooted, it
wouldn't have changed the system mtu back to 1500 unless someone
entered the system mtu 1500 command and then rebooted. Am I
understanding that correctly?
On Apr 11, 2005 9:06 PM, marvin greenlee <marvin@ccbootcamp.com> wrote:
> If a system MTU change on the router requires a reboot in order to take
> effect, don't you think the MTU change would be able to survive a reboot?
> Look at the flash before and after a MTU change.
>
> **** Before changing ****
>
> 3550_B#show flash
>
> Directory of flash:/
>
> 2 -rwx 4203904 Jan 1 1970 09:27:03 +00:00
> c3550-i5q3l2-mz.121-20.EA1
> a.bin
> 3 -rwx 976 Mar 6 1993 00:49:18 +00:00 vlan.dat
> 5 -rwx 0 Mar 25 1993 07:14:00 +00:00 system_env_vars
> 6 -rwx 5 Mar 1 1993 06:31:15 +00:00 private-config.text
> 7 -rwx 3191 Mar 1 1993 06:31:15 +00:00 config.text
> 8 -rwx 16 Mar 25 1993 07:14:00 +00:00 env_vars
>
> 15998976 bytes total (11788288 bytes free)
>
> **** Set the clock, and change MTU ****
>
> 3550_B#clock set 12:00:00 11 April 2005
> 3550_B#conf t
> Enter configuration commands, one per line. End with CNTL/Z.
> 3550_B(config)#system mtu 1505
>
> **** After changing ****
>
> 3550_B#show flash
>
> Directory of flash:/
>
> 2 -rwx 4203904 Jan 1 1970 09:27:03 +00:00
> c3550-i5q3l2-mz.121-20.EA1
> a.bin
> 3 -rwx 976 Mar 6 1993 00:49:18 +00:00 vlan.dat
> 4 -rwx 16 Apr 11 2005 12:00:16 +00:00 env_vars
> 6 -rwx 5 Mar 1 1993 06:31:15 +00:00 private-config.text
> 7 -rwx 3191 Mar 1 1993 06:31:15 +00:00 config.text
> 8 -rwx 0 Apr 11 2005 12:00:16 +00:00 system_env_vars
>
> Marvin Greenlee, CCIE#12237, CCSI# 30483
> Network Learning Inc
> marvin@ccbootcamp.com
> www.ccbootcamp.com (Cisco Training)
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Matt
> Mullen
> Sent: Monday, April 11, 2005 5:22 PM
> To: James Ventre
> Cc: Group Study
> Subject: Re: system MTU and OSPF [bcc][faked-from]
> Importance: Low
>
> James,
>
> Yes, I did have 802.1Q tunneling configured on interface f0/20.
> According to the doc CD for 802.1Q tunneling the system MTU is 1500 by
> default and you must manually set it to 1504 with the 'system mtu'
> command. I didn't configure the 'system mtu' command however. One
> possibility I just thought of was that maybe someone on the rack
> before me set the system mtu to 1504 and the switch wasn't rebooted.
>
> Thanks,
> Matt
>
> On Apr 11, 2005 7:03 PM, James Ventre <messageboard@ventrefamily.com> wrote:
> > >1) Why was the MTU of the switch appearing as 1504?
> >
> > Do you have 802.1Q tunneling on? I'm not sure if it would increase it
> > by default, but that would require 4 extra bytes in the MTU.
> >
> > James
> >
> >
> > Matt Mullen wrote:
> >
> > >Hello group,
> > >
> > >I was working a lab scenario this past weekend where there were two
> > >routers, R6 and R5 both trying to form an OSPF adjacency to a 3550
> > >switch via a SVI (VLAN interface). The adjacencies were not coming
> > >up, and a 'debug ip ospf adj' showed me that there was an MTU
> > >mismatch between the routers and the 3550 switch. The switch had an
> > >MTU of 1504 while the routers had an MTU of 1500. To correct the
> > >problem, I set the MTU on the 3550 switch SVI to 1500.
> > >
> > >Today going back and checking my work against the solutions, the
> > >solutions show 'ip ospf mtu-ignore' being configured on the routers
> > >and on the SVI. So, this leads me to a couple of questions:
> > >
> > >1) Why was the MTU of the switch appearing as 1504? Is it possible
> > >that the switch automatically bumped the system mtu up to 1504 because
> > >I had previously configured 802.1Q tunneling? I thought you had to
> > >manually set the 'system mtu 1504' for this to happen, which I didn't
> > >do.
> > >
> > >2) Is there anything wrong with the solution I used to solve the
> > >problem? i.e. Would I get the points?
> > >
> > >TIA,
> > >
> > >Matt
> > >
> > >_______________________________________________________________________
> > >Subscription information may be found at:
> > >http://www.groupstudy.com/list/CCIELab.html
> >
> > _______________________________________________________________________
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>
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