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

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?

—ovs-vsctl show
5b154321-534d-413e-9761-60476ae06640
    Bridge "cloudbr0"
        Port "cloudbr0"
            Interface "cloudbr0"
                type: internal

—MTU from the bridge after set an MTU into the XML File (before we had here
9000)
mtu                 : 2000
mtu_request         : []
name                : "cloudbr0"
ofport              : 65534

—MTU of the vnet interface
mac_in_use          : "fe:00:74:76:00:01"
mtu                 : 1450
mtu_request         : 1450
name                : „vnet2"

Thanks for help…



__

Sven Vogel
Teamlead Platform

EWERK RZ GmbH
Brühl 24, D-04109 Leipzig
P +49 341 42649 - 11
F +49 341 42649 - 18
S.Vogel@ewerk.com
www.ewerk.com

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Registergericht: Leipzig HRB 17023

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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

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


__

Sven Vogel
Teamlead Platform

EWERK RZ GmbH
Brühl 24, D-04109 Leipzig
P +49 341 42649 - 11
F +49 341 42649 - 18
S.Vogel@ewerk.com
www.ewerk.com

Geschäftsführer:
Dr. Erik Wende, Hendrik Schubert, Frank Richter
Registergericht: Leipzig HRB 17023

Zertifiziert nach:
ISO/IEC 27001:2013
DIN EN ISO 9001:2015
DIN ISO/IEC 20000-1:2011

EWERK-Blog | LinkedIn | Xing | Twitter | Facebook

Auskünfte und Angebote per Mail sind freibleibend und unverbindlich.

Disclaimer Privacy:
Der Inhalt dieser E-Mail (einschließlich etwaiger beigefügter Dateien) ist
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bestimmungsgemäße Empfänger sein, ist Ihnen jegliche Offenlegung,
Vervielfältigung, Weitergabe oder Nutzung des Inhalts untersagt. Bitte
informieren Sie in diesem Fall unverzüglich den Absender und löschen Sie die
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Dank.

The contents of this e-mail (including any attachments) are confidential and may
be legally privileged. If you are not the intended recipient of this e-mail, any
disclosure, copying, distribution or use of its contents is strictly prohibited,
and you should please notify the sender immediately and then delete it
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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