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Re: [Xen-devel] [PATCH] set mtu from bridge also on vif interface

To: Olaf Hering <olaf@xxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] set mtu from bridge also on vif interface
From: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Date: Mon, 7 Feb 2011 09:36:32 +0000
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On Mon, 2011-02-07 at 09:25 +0000, Olaf Hering wrote:
> On Mon, Feb 07, Ian Campbell wrote:
> > On Sun, 2011-02-06 at 13:41 +0000, Olaf Hering wrote:
> > > Apply mtu size from bridge interface also in vif interface.
> > > This depends on a kernel change which allows arbitrary mtu sizes until
> > > the frontend driver has connected to the backend driver. Without this
> > > kernel change, the vif mtu size will be limited to 1500 even with this
> > > change to the vif-bridge script.
> > > 
> > > Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> > 
> > In my experience the bridge will only allow it's MTU to be set to the
> > minimum of all the ports (or 1500ish if there are no ports) and will
> > reset to the new minimum when you add a new port.
> > 
> > So does this patch do anything in the absence of the ability to
> > configure the MTU of a VIF from the toolstack?
> This depends if the bridge interface is part of the Xen toolstack
> configuration or if the bridge is part of the system configuration. In
> my setup the bridge is part of the system configuration and gets its mtu
> from the config of the physical interface, eth0 in my case. So br0 has
> mtu of 9000 already when vif-bridge runs.

When I was playing with this last week I was using an internal bridge
(i.e. no physical interface, because I have no physical jumbo
infrastructure) so the physical interface didn't force the bridge.

> With this change, the vif interfaces follow the system configuration.

ok, that's cool then.


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