From: Victor Cappuccio (victor@ccbootcamp.com)
Date: Fri Jun 29 2007 - 03:10:26 ART
for example:
from CCIE Practical Studies Volume II
The default system maximum transmission unit (MTU) for traffic on the Catalyst
3550 switch is 1500 bytes. The 802.1Q tunneling feature increases the frame
size by 4 bytes. Therefore, when you configure 802.1Q tunneling, you must
configure all switches in the 802.1Q network to be able to process maximum
frames by increasing the switch system MTU size to at least 1504 bytes. You
configure the system MTU size by using the system mtu global configuration
command.
thanks,
Victor Cappuccio.-
- CCSI# 31452
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-----Original Message-----
From: nobody@groupstudy.com on behalf of Chris Proctor
Sent: Fri 6/29/2007 5:19
To: John Gibson; Mark Mckillop(mmckillo); Antonio Soares; Radioactive Frog
Cc: ccielab@groupstudy.com
Subject: RE: RE: MTU Mismatch in OSPF
Does anyone know where the 1504 byte is coming from? That doesn't even
ring a bell as a standard MTU + encapsulation match (IP + MPLS shim
maybe?) Is it just a code bug or what?
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
John Gibson
Sent: Sunday, June 24, 2007 2:46 PM
To: Mark Mckillop (mmckillo); Antonio Soares; Radioactive Frog
Cc: ccielab@groupstudy.com
Subject: RE: RE: MTU Mismatch in OSPF
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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