libvirt users - Jul 2019 - Re: OVS / KVM / libvirt / MTU

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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> Am 29.07.2019 um 13:20 schrieb Michal Privoznik
<mprivozn@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

On 7/29/19 9:23 PM, Sven Vogel wrote:
> 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?
I don't know enough about OVS internals to answer that, sorry. Maybe we 
should ask OVS developers why OVS bridge behaves this way.

Michal

In general if no MTU is set on interface creation the default value is 1500.

On OVS the bridge MTU is automatically set to the smallest port MTU. So 
you just have to set the MTU of each port of the bridge.

Take a look at: https://bugzilla.redhat.com/show_bug.cgi?id=1160897
It is a bit of a pain to read but seems to confirm the statement about 
the OVS MTU value being set by the MTU of the ports.

I have been using OVS with Xen and a few years ago I had to wrap my head 
around this problem.
Once you start setting the MTU to something other than 1500 you will 
find you need to set ALL the MTU values of ALL your interfaces.
Otherwise some inherited default will bite you.


On 7/31/19 4:39 AM, Michal Privoznik wrote:
> On 7/29/19 9:23 PM, Sven Vogel wrote:
>> 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?
>
> I don't know enough about OVS internals to answer that, sorry. Maybe 
> we should ask OVS developers why OVS bridge behaves this way.
>
> Michal
>
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