From: Richard Davidson (rich@myhomemail.net)
Date: Wed Jun 15 2005 - 13:17:52 GMT-3
When the frame is switched out the serial interface,
the serial interface has a maximum transmit size. If
the frame is larger than the MTU of that interface,
the frame will be fragmented.
However, assuming the source is from Ethernet
(1500Byte MTU) the serial will never fragment: The
Ethernet node will fragment 3000Byte IP packet before
its ingress as a frame into the transceiver and onto
the Ethernet network. When the frame arrives at the
Ethernet router, it is already >=1500Bytes, fragmented
by the originating node and the router will do
nothing.
If the switch connecting to the node saw a > 1500 byte
frame, I think it is actually 1518, but it may discard
it as a giant frame.
Richard Davidson
--- ccie2be <ccie2be@nyc.rr.com> 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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