Xen and VLANs

Xen and VLANs

[Evan Bigall]

I am using Xen 2.0 with Mandrake 10.1 based hosts and guests.

There seems to be some sort of issue with VLANs and MTUs?

My VMs have two nics, each connected to a separate bridge.

In Dom1, I do: 

vconfig add eth1 5
ip addr add 10.20.8.17/24 broadcast 10.20.8.255 dev eth1.5
ifconfig eth1.5 up

In Dom2, I do:

vconfig add eth1 5
ip addr add 10.20.8.18/24 broadcast 10.20.8.255 dev eth1.5
ifconfig eth1.5 up

Pinging from Dom2 to Dom1, packet sizes smaller than 1469 work, packet sizes of 1469 and larger just hang.

If I set the MTU on the interface down to 1450, I can then ping with any packet size.

Has any done any experimentation with VLANs on Xen?  A problem is that the bridge does not support VLANs but I'm trying to work around that in a variety of ways.

Evan



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RE: Xen and VLANs

[Ian Pratt]

> I am using Xen 2.0 with Mandrake 10.1 based hosts and guests.
> 
> There seems to be some sort of issue with VLANs and MTUs?

The interface in your dom0 can't support the normal ethernet mtu because
of the VLAN headers. We need to add a way of allowing the backend vif to
signal this to the frontend so that the maximum MTU is adjusted
appropriately. 

This is already planned for Xen 3.0. For the moment, I'm afriad you'll
need to reduce the MTU manually in the guest.

Ian



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Re: Xen and VLANs

[David Becker]

" Pinging from Dom2 to Dom1, packet sizes smaller than 1469 work, packet 
" sizes of 1469 and larger just hang.

I'd guess the bridging doesn't account for the space used by the vlan tag.

" Has any done any experimentation with VLANs on Xen?

Yes, I did the vconfig in dom0 and attached the vlan interface to the
bridge.  This way no vlan tags go over the bridge.
If you really want to see the tags in xenU, this scheme will not help.
    dom0# vconfig eth0 145
    dom0# brctl xen-br0 addif eth0.145



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Re: Xen and VLANs

[Fajar A. Nugraha]

Evan Bigall wrote:

> I am using Xen 2.0 with Mandrake 10.1 based hosts and guests.
>
> There seems to be some sort of issue with VLANs and MTUs?
>
Why bother use VLANS in dom-U?

> My VMs have two nics, each connected to a separate bridge.
>
> In Dom1, I do:
> vconfig add eth1 5
> ip addr add 10.20.8.17/24 broadcast 10.20.8.255 dev eth1.5
> ifconfig eth1.5 up
>
> In Dom2, I do:
>
> vconfig add eth1 5
> ip addr add 10.20.8.18/24 broadcast 10.20.8.255 dev eth1.5
> ifconfig eth1.5 up
>
> Pinging from Dom2 to Dom1, packet sizes smaller than 1469 work, packet 
> sizes of 1469 and larger just hang.
>
> If I set the MTU on the interface down to 1450, I can then ping with 
> any packet size.
>
> Has any done any experimentation with VLANs on Xen?  A problem is that 
> the bridge does not support VLANs but I'm trying to work around that 
> in a variety of ways.
>
I use bridge, setup VLANS on Domain-0, and bridge each eth0.x on 
Domain-0 to ethy on dom-U.
That way dom-U only sees eth0, eth1, eth2, and so on. It works flawlessly.

Regards,

Fajar


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