From: Matt Mullen (mullenm@gmail.com)
Date: Mon Apr 11 2005 - 21:22:14 GMT-3
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
> >
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