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Date:	Wed, 29 Oct 2008 16:24:51 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	netdev <netdev@...r.kernel.org>
Subject: linux bridge and MTU

There's an interesting interaction between different
MTU (max transmission unit) values on interfaces
which are bridged together.  I'm trying to understand
how it works.

Suppose there are 2 interfaces in the bridge, one is
with standard 1500 mtu and another is, say 3500.

As far as I can see, bridge interface sets its mtu to
be the smallest of all the components.  Which seems
to be the right ting to do.

But now the question is - is it possible to communicate
over the interface with larger MTU using full frames?

For example, here are a tcpdump from a single ping-pong
"pair" between host "B" which is connected to a larger-MTU
interface, and host "A" which is with the bridge described
above, using 3000-byte packets:

IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ICMP (1), length 3028) B > A ICMP echo request, id 35331, seq 2, length 3008

IP (tos 0x0, ttl 64, id 39747, offset 0, flags [+], proto ICMP (1), length 1500) B > A: ICMP echo reply, id 35331, seq 2, length 1480
IP (tos 0x0, ttl 64, id 39747, offset 1480, flags [+], proto ICMP (1), length 1500) B > A: icmp
IP (tos 0x0, ttl 64, id 39747, offset 2960, flags [none], proto ICMP (1), length 68) B > A: icmp

So, the reply comes in 3 packets according to 1500 MTU of
the bridge interface.

When forwarding from B to some host C connected to the other
interface with standard 1500 mtu, host A correctly sends
"fragmentation required" ICMP back, so that part works.
Also, host A obviously is able to receive larger frames.
But it can't SEND larger frames, even if the underlying
interface has proper MTU settings?

Is there a way to achieve this?

Thanks!

/mjt
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