From: Richard Gallagher (rgallagh@cisco.com)
Date: Wed Jun 15 2005 - 10:19:20 GMT-3
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
>
>_________________________________________________________________
>Express yourself instantly with MSN Messenger! Download today - it's FREE!
>http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/
>
>_______________________________________________________________________
>Subscription information may be found at:
>http://www.groupstudy.com/list/CCIELab.html
>
>_______________________________________________________________________
>Subscription information may be found at:
>http://www.groupstudy.com/list/CCIELab.html
This archive was generated by hypermail 2.1.4 : Wed Jul 06 2005 - 14:43:41 GMT-3