Re: [Xen-users] Jumbo Frames in Xen 3.3

[Reto Gantenbein <reto.gantenbein@xxxxxxxxxxx>]

Hi everybody

It's basically possible to use a higher MTU than 1500 but with some  
constraints. We have a setup for using MTU 5000 for our domUs.
The procedure is as follows:
- enable MTU 5000 on the physical interface of dom0
- enable MTU 5000 on the interface of domU
- after domU startup, enable MTU 5000 at the virtual interface (vif)  
of that domU in dom0
- the Xen bridge still shows MTU 1500. The MTU cannot be set for  
bridges but afaik they also don't care about MTU size.
The constraints are:
- MTU 5000 doesn't work between two domUs inside the same dom0
- MTU 5000 doesn't work reliable between two domUs inside different  
dom0s (may be issue with our iptables setup?!)
- MTU 5000 works reliable between domUs and physical servers

I hope this helps you.

It would be very nice to see native jumbo frame support in Xen in the  
future. We currently use Xen-3.2.0 but I also couldn't find anything  
related to this topic in the Xen-3.3.0 changelog. So I filled a bug  
at: http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1341
Kind regards
Reto Gantenbein


On Mar 7, 2008, at 10:54 AM, Pasi Kärkkäinen wrote:

> On Thu, Mar 06, 2008 at 04:24:25PM +0000, Gareth Bult wrote:
> > Hi,
> >
> > After failing to make Jumbo frames work on my Xen installation, I  
> > read that Xen doesn't support Jumbo Frames.
> What did you actually do/configure when you tried jumbo frames?
>
> > Is this really true?
> I'm not sure.. I have never really tried jumbo frames with Xen..
>
> I assume first you need to configure your ethX (pethX) devices to  
> use 9000
> bytes mtu.. and then your bridge devices to use 9000 mtu too.
>
> After that you need to configure your ethX devices in domU to use  
> 9000 mtu.
> Is that enough? Has someone tried this?
>
> -- Pasi
>
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