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RE: [Xen-devel] vif limitation to 1500 bytes (no vlan ?)

To: "Yann Dupont" <Yann.Dupont@xxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] vif limitation to 1500 bytes (no vlan ?)
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Fri, 22 Jul 2005 12:56:53 +0100
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Thread-topic: [Xen-devel] vif limitation to 1500 bytes (no vlan ?)

> I have a small problem; Is there a known limitation on vif 
> code - Network-device backend or frontend - which prohibits 
> the usage of tagged
> (802.1q) frames inside a domU ?
> In dom0 I have a bridge with eth0 (epro1000) wich is 
> connected to a 802.1q port this same bridge has also a vif 
> connected, corresponding to a domU.
> Inside this domU , I can use vconfig to have numerous 
> interfaces derived from the vif, one by vlan, BUT can only 
> safely use a MTU of 1496. If I use the default MTU I have problems.
> For the moment I have solved the problem by using the 
> physicall interface INSIDE the domU. But I'd like to know if 
> this problem is known ?

The frontend/backend itself should work for any MTU up to something shy
of 4KB.

I suspect the 1496 restriction is actually being introduced in dom0 by
the 802.1q tagging, but this information is not being propagated to the
frontend in the domU.
Because this is a bridged rather than routed setup, MTU discovery won't

I've a feeling that an 'mtu=' parameter might have been added a while
back so that you could fix the MTU of a frontend vif in the domain
config file. We could figure this out automatically by looking at the
bridge the vif is connected to and then pick the lowest MTU of any
device connected to the bridge. Patches welcome.


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