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

[Sven Vogel]

Hi Michal,

Thanks for your answer.

I don’t understand why an interface created without mtu gets only 1500 visible in the virtual machine but if I create an interface with mtu higher than 1500 e.g. 2000 the bridge will change too. Before the bridge was e.g. by 9000.
I ask because you wrote if I don’t set an mtu of the interface I will get the mtu of the bridge. But it seems so.

Can you clarify it a little better for me?


Cheers

Sven

Von meinem iPhone gesendet


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Sven Vogel
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> Am 29.07.2019 um 13:20 schrieb Michal Privoznik <mprivozn at redhat.com>:
>
>> 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