From: Smith, Jason (JASmith@nuvox.com)
Date: Tue Jun 26 2007 - 10:02:04 ART
I recently had this problem and it took a while to figure it out. Yes,
the switches when set as a dot1q trunk will add on an extra 4 bytes in
the payload to buffer it. The problem comes up when you have for
instance an ONS running a E100 card with a dot1 connection to you. But
when it dumps the VLAN off for customers without a dot1q trunk the
packet is a runt without that extra 4 bytes which is why it adds that
packet. Routers on the other hand, for instance a 7200 connected in the
same way will not pad the extra 4 bytes and will cause issue on the
customer vlans. Here is a link explaining it a little more.
http://www.cisco.com/en/US/partner/products/hw/optical/ps2006/products_t
ech_note09186a008020ab22.shtml
Thanks,
Jason Smith
CCIE #12097
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Paul Cosgrove
Sent: Tuesday, June 26, 2007 5:35 AM
To: John Gibson
Cc: ccielab@groupstudy.com
Subject: Re: MTU Mismatch in OSPF
Hi John,
While the MTU is normally listed as 1500, I think cisco look on this as
an MTU per vlan. The switches allow an additional 4 bytes for a single
dot1q tag to be added so that the frames can traverse trunk links. I'm
sure there is a reference hidden away somewhere in the docs but can't
find it now. Would be interesting to send some test packets through the
switch and see if this is a real change or just a different way of
presenting existing behaviour.
Paul.
John Gibson wrote:
> I am thinking that if the switches have mtu 1504
> and routers have 1500, when the switches tries
> to initiate TCP connections (e.g. ftp download
> some image files) to the routers, the packet will
> , by default without extra protection mechanism,
> be dropped at some router.
>
> But TCP has a MSS that is around 1460 (with my
> linux box, and I hope this is the same in the catalyst
> switches) , which limits the IP packet size to
> 1500 when the OS tries to make packets. So FTP
> download will be fine.
>
> But I am thinking that TFTP download will fail.
> UDP does not have MSS.
>
> John
>
> --- "Mark Mckillop (mmckillo)" <mmckillo@cisco.com>
> wrote:
>
>
>> A nice option also is to do:
>>
>> system mtu routing 1500
>>
>> This will avoid you needing to use the mtu-ignore
>> command, although it
>> only works on the 3560's. It doesn't require a
>> reboot either. Just
>> ignoring something always feels like a bad thing
>> thing to do :)
>>
>> Mark.
>>
>> -----Original Message-----
>> From: nobody@groupstudy.com
>> [mailto:nobody@groupstudy.com] On Behalf Of
>> Antonio Soares
>> Sent: 23 June 2007 17:58
>> To: 'Radioactive Frog'; johngibson1541@yahoo.com
>> Cc: ccielab@groupstudy.com
>> Subject: RE: RE: MTU Mismatch in OSPF
>>
>> +++++++++++++++++++++++++++++++++++++++++++++++++++
>> The 3560 default is 1500:
>>
>> Switch#sh flash
>>
>> Directory of flash:/
>>
>> 2 -rwx 7963038 Mar 1 1993 00:57:27 +00:00
>> c3560-advipservicesk9-mz.122-25.SEE2.bin
>>
>> 32514048 bytes total (24549888 bytes free) Switch#
>> Switch# Switch#sh
>> flash
>> +++++++++++++++++++++++++++++++++++++++++++++++++++
>> Directory of flash:/
>>
>> 2 -rwx 7963038 Mar 1 1993 00:57:27 +00:00
>> c3560-advipservicesk9-mz.122-25.SEE2.bin
>>
>> 32514048 bytes total (24549888 bytes free)
>>
>> Switch#sh system mtu
>>
>> System MTU size is 1500 bytes
>> System Jumbo MTU size is 1500 bytes
>> Routing MTU size is 1500 bytes
>> Switch#
>> +++++++++++++++++++++++++++++++++++++++++++++++++++
>>
>> But the way, when you have an OSPF Mtu Mismatch,
>> where you do add the
>> "ip ospf mtu-ignore" command ?
>>
>> 1) The device with the lower Mtu.
>> 2) The device with the higher Mtu.
>> 3) Both devices.
>>
>>
>>
>>
>> -----Original Message-----
>> From: nobody@groupstudy.com
>> [mailto:nobody@groupstudy.com] On Behalf Of
>> Radioactive Frog
>> Sent: sabado, 23 de Junho de 2007 5:23
>> To: johngibson1541@yahoo.com
>> Cc: ccielab@groupstudy.com
>> Subject: Re: RE: MTU Mismatch in OSPF
>>
>> dats sound strange to me!
>> frog
>>
>>
>> On 6/23/07, johngibson1541@yahoo.com
>> <johngibson1541@yahoo.com> wrote:
>>
>>> I think 3550's MTU at 1504 is caused by a file
>>>
>> flash:env_vars
>>
>>> If I remove this file, then reload it will become
>>>
>> 1500.
>>
>>> My 3560 is a different story. It is 1504 by
>>>
>> itself.
>>
>>> Strangely, univercd keeps talking about 3560 and
>>>
>> 3550 default mtu
>>
>>> being 1500. They are just not saying the correct
>>>
>> thing.
>>
>>> 3560 at 12.2(25) , 12.2(35) , and the latest
>>>
>> 12.2(37) all make the
>>
>>> same mistake!
>>>
>>>
>>>
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