netdev - Fw: Subnet broadcast MTU issue?

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Message-ID: <623F3F55DDA14658A341954348829D1A@GerritRita>
Date:	Mon, 10 May 2010 21:51:21 +0200
From:	"Gerrit Binnenmars" <gerritbinnenmars@...il.com>
To:	<netdev@...r.kernel.org>
Subject: Fw: Subnet broadcast MTU issue?

Hello,

I have configured an ethernet port at MTU 9000 with ifconfig
On this port I have an alias (with IP 192.168.100.254) for a network that 
support only an MTU of 1500, I added ip route change 192.168.100.0/24 dev 
eth0 src 192.168.100.254 mtu lock 1500.

Then ping -s 20000 192.168.100.253 works fine, with packets being fragmented 
as expected at 1500 bytes
but ping -b -s 20000 192.168.100.255 does not work, packets are fragmented 
at approx. 9000 bytes.

I tried ip route change table local broadcast 192.168.100.255 mtu lock 1500 
without success.

Is there a solution? Where in the source code is the MTU size for a 
broadcast message determined?

Thanks in advance,

Gerrit Binnenmars 

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