[libvirt-users] OVS / KVM / libvirt / MTU

[Michal Privoznik]

On 7/23/19 8:00 PM, Sven Vogel wrote:
> Hey there,
> 
> I hope anyone can bring some light in the following problem.
> 
> <interface type='bridge'>
> <source bridge='cloudbr0'/>
> <mtu size='2000'/>
> <mac address='02:00:74:76:00:01'/>
> <model type='virtio'/>
> <virtualport type='openvswitch'>
> </virtualport>
> <vlan trunk='no'>
> <tag id='2097'/>
> </vlan><link state='up'/>
> </interface>
> 
> we have an base bridge for example and cloudbr0. After we add an mtu to the vm bridge here it seems the base bridge gets the same mtu like the vnet adapter.
> 
> Is this normal behaviour of libvirt together with OVS?

I wouldn't say it is 'normal', but this behaviour is not specific to 
libvirt. I mean, if you create a tap device yourself by hand and then 
plug it into an ovs bridge the same thing will happen. If you don't want 
to change MTU of the bridge then just don't set it for <interface/> and 
libvirt will copy whatever MTU the bridge has over to the new TAP device.

Michal