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RE: [Xen-devel] Ethernet MTU

To: "'Adrian Chadd'" <adrian@xxxxxxxxxxxxxxx>
Subject: RE: [Xen-devel] Ethernet MTU
From: "Sylvain Coutant" <sco@xxxxxxxxxx>
Date: Wed, 16 Aug 2006 14:30:59 +0200
Cc: 'xen-devel' <xen-devel@xxxxxxxxxxxxxxxxxxx>
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> > Is there any plan to raise supported ethernet mtu above 1500 ?
> Currently, packets are dropped above this point (tested against xen-3.0.2-
> testing). Even 802.1q tags require explicitly lowering the mtu to 1496 to
> work.
> Keeping this -just- on xen-devel -
> Which interface are you talking about being > 1500 bytes?

My primary point is to support jumbo frames (9000 bytes mtu). My second point 
is to trunk domU to an external switch.

1/ I am able to set up a linux bridge with 9000 bytes mtu without trouble as 
long as it does not involve any Xen interface. And unable to set the mtu on 
vifs to something larger than 1500 bytes. I would like to configure my vifs to 
be compliant in mtu with the Ethernet network they are connected to which is 
full jumbo.

2/ I tried several scenario such as managing tags in dom0's (and setup as many 
bridges as vlans) or trunking vlan frames to domUs. In this later case, I need 
to bring down the mtu to 1496 for the frames to be forwarded across the bridge. 
Whatever code is rejecting the frames, it could be avoided if Xen's vifs were 
able to support larger mtus. 



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