[virt-tools-list] [RFC] enable direct interface selection

Gerhard Stenzel gstenzel at linux.vnet.ibm.com
Tue Apr 5 14:54:49 UTC 2011

On Tue, 2011-04-05 at 09:58 -0400, Cole Robinson wrote:
> I'm having difficulty decoding this. What is the functional difference
> between these two modes? Can both modes talk to network, host, and
> host
> VMs? The bridge caveat that both devices be in bridge mode, if that
> isn't the case, can packets not be delivered, or are they just
> delivered
> sub-optimally? 

In vepa mode all packets are sent to the external switch. Normally, a
switch will not send a packet back to the same port where it came from.
So, a guest using a direct interface in vepa mode can not talk to
another guest or the host using the same interface.
If they need to talk to each other, the switch must support "hair pin"
mode where the switch sends the packet back to the same port. For
example, a linux based switch can be configured for "hair pin" mode.

In bridge mode, guests using the same interface will be able to talk
directly, that is the packets will not be send to the external switch.
To talk to the host, the host interface will need to be reconfigured in
the following way:

sudo ip link add link eth0 type macvlan mode bridge
sudo ifconfig eth0
sudo ifconfig macvlan0 netmask up

I hope this clarifies more than it confuses.

Best regards, 

Gerhard Stenzel, 
IBM Deutschland Research & Development GmbH
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