From: John Matus (jmatus@pacbell.net)
Date: Thu Jun 16 2005 - 00:54:47 GMT-3
ok, i'm going to have to plead ignorance on this. you say there is
interface mtu and protocol mtu<?>. i've configured mtu on an interface but
withe the "ip mtu" command.........so it sounds like that would be protocol
mtu.......but then what would be the command for interface mtu???
Regards,
John D. Matus
MCSE, CCNP
Office: 818-782-2061
Cell: 818-430-8372
jmatus@pacbell.net
----- Original Message -----
From: "Richard Gallagher" <rgallagh@cisco.com>
To: "ccie2be" <ccie2be@nyc.rr.com>; "Group Study" <ccielab@groupstudy.com>
Sent: Wednesday, June 15, 2005 6:19 AM
Subject: Re: system mtu
> Cisco routers don't have a configurable "system mtu", this option appears
> to be avaible only on certain switches eg 2950, 35xx etc....on these
> platforms interfaces are generally of the same type and therefore
> thereshould be no need for different mtu, added to that you generally
> can't configure the interface mtu on these platforms. Try this on a 3550
> and you will see.
>
> Most routers just have a configurable interface mtu and then protocol mtu
> (ip, mlps, ipx etc....), so if in your example a 3k byte packet came in on
> an interface with a 10k and havd to be forwarded out of interface with
> 1.5k then the router will fragment the packet.
>
> You'll should be able to see this if you enable "debug ip packet detail"
> on a lab router - of course unless the DF bit is set :)
>
> HTH, Rich
>
> ccie2be wrote:
>
>>John,
>>
>>Suppose a router had a bunch of serial interfaces with an MTU of 10,000
>>and
>>an ethernet interface with an MTU of 1500.
>>
>>And, let's say the system MTU was set to 10,000 and the eth interface MTU
>>was set to 1500.
>>
>>Now, the router wants to send a packet of size 3000 out the ethernet
>>interface.
>>
>>Based on the system MTU, no fragmentation is needed. What does the router
>>do?
>>
>>Since it's not possible to send a 3000 byte packet out an ethernet
>>interface
>>with a max MTU of 1500, I would say the router will fragment that packet.
>>
>>HTH, Tim
>>
>>-----Original Message-----
>>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>>John
>>Matus
>>Sent: Wednesday, June 15, 2005 12:32 AM
>>To: ccielab@groupstudy.com
>>Subject: system mtu
>>
>>question regarding mtu on an interface and globally and how the 2 interact
>>with each other...
>>i think i know part of it but i don't know which direction it works..
>>
>>is it - if the interface exceeds it's own mtu, then it will fragment
>>packets in accordance with the system mtu? or is it the other way
>>around....
>>or....
>>
>>is the global mtu the default for all interfaces and the interface mtu
>>just overrides the global setting?
>>
>>TIA
>>
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