Re: [Xen-users] Changing MTU of vif

[Tracy R Reed <treed@xxxxxxxxxxxxxxx>]

James Harper wrote:
> I've been vaguely following this discussion, and AFAIK, xen does not
> provide a mechanism to keep the backend and frontend MTU's in sync.
This isn't really what I'm asking for although it would be nice. I just 
want to automatically set the MTU on the vif when the domain is created. 
I can setup my network scripts inside the domain to hard code eth1 
(always my SAN interface in a domU) to MTU 9000.
> Possibly it would only be legal to change the MTU value while the
> interface was not in the "connected" state...
I have noticed that I cannot change the MTU on the bridge when other 
devices are attached with a different MTU. I can set the MTU of the vif 
attached to the bridge to 9000. Only after setting the vif to 9000 can I 
set the bridge to 9000. Then I also have to set the MTU on eth1 in the 
domU to 9000. Then everything works perfectly. I can set MTU=9000 in the 
/etc/sysconfig/network-scripts/ifcfg-eth1 file. But I need to find a way 
to automate setting MTU to 9000 on the bridge and vif.
I notice that if I create a domU and do all of the above and get the MTU 
on everything set to 9000 and then I create another domU the MTU on the 
bridge gets set back to 1500 and the new vif is also 1500. The first vif 
created on the SAN bridge still has an MTU of 9000. So now I have to set 
the new vif to 9000, then I can set the bridge back to 9000 and 
everything works fine again until I add another domU at which point I 
have to do this dance again.
> Does anyone know if, on a linux bridge, there is any reason why all
> attached devices wouldn't have the same MTU?
I can't think of any reason. It would seem to make sense.

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