Fw: [Bugme-new] [Bug 4430] New: Virtual interfaces cannot have their own mtu

Fw: [Bugme-new] [Bug 4430] New: Virtual interfaces cannot have their own mtu

[Andrew Morton]

hm, mtu is implemented in the device driver - you might be out of luck.


Begin forwarded message:

Date: Fri, 1 Apr 2005 02:01:19 -0800
From: bugme-daemon@osdl.org
To: bugme-new@lists.osdl.org
Subject: [Bugme-new] [Bug 4430] New: Virtual interfaces cannot have their own mtu


http://bugme.osdl.org/show_bug.cgi?id=4430

           Summary: Virtual interfaces cannot have their own mtu
    Kernel Version: kernel-2.6.9-1.6_FC2
            Status: NEW
          Severity: low
             Owner: acme@conectiva.com.br
         Submitter: lukeross@sys3175.co.uk


Distribution: Fedora Core 2,3
Hardware Environment: Broadcom gigabit card using tg3 (Tyan s2885 onboard)
Problem Description:

eth0 and eth0:1 cannot have different mtus. I have a jumbo-frame capable switch
with three devices plugged in. Two are PCs with jumbo-capable cards, the other
is a wireless router which isn't, and hangs if either PC attempts to discover
whether it can support jumbo frames.

To get the benefit of jumbo frames between the two PCs, I tried to set up eth0:1
- on a different subnet to the wireless router - on both PCs, and set the mtu of
the eth0:1 to 9000. However it is not possible to set the mtu for eth0:1 to 9000
without setting the mtu of eth0 to 9000 as well.

Also noted in http://xcat.org/pipermail/xcat-user/2003-April/002358.html

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Re: Fw: [Bugme-new] [Bug 4430] New: Virtual interfaces cannot have their own mtu

[Herbert Xu]

Andrew Morton <akpm@osdl.org> wrote:
>
> the eth0:1 to 9000. However it is not possible to set the mtu for eth0:1 to 9000
> without setting the mtu of eth0 to 9000 as well.

The solution is to set the mtu using ip route in addition to setting
it on eth0, e.g.,

ip ro add x.0.0.0/8 via gw dev eth0 mtu 1500 src a.b.c.d
ip ro add y.0.0.0/8 via gw2 dev eth0 mtu 9000 src e.f.g.h

You still have to set the mtu on eth0 to 9000 since that determines
the maximum receive size as well (MRU).
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