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Re: [libvirt-users] vnet device not appearing

On 05/28/2014 10:25 AM, Liam MacKenzie wrote:

I have a virtual server with 3 existing VMs running on it without issue.  The host has 4 NICs installed; em1 for the host, em2 and em3 already in use by other VMs. 
I would like to dedicate em4 to my new VM however when I create the bridge like I did for the others I do not get a vnet3 device show up in the list as shown below.

I think you are misunderstanding how the vnetX devices come into existence. They are created automatically when a guest is *started*; they are *not* created when the guest is being configured and you should not add a reference to a vnet device anywhere in your config. Rather, you should select the *bridge device* for the guest's network connection. (It is actually a bug that virt-manager displays the vnetX devices in the connection list at all; I had thought that they were now removed...)

Beyond that, I'm wondering if you really need to completely dedicate a physical network device to a single guest - once you have a bridge device, you can connect up to 255 (or maybe 254? I've never gone to the limit) guests to a single bridge. Of course they share the bandwidth, but this may be what's happening at your switch anyway (or they may not have enough traffic for it to matter). So you could save yourself the trouble of plugging in so many network cards and give yourself more felxibility by just having a single bridge connected to a single ethernet, and connect all of your guests to that one bridge.

Alternately, if you want to dedicate each physical card to a single guest (and not use it for host networking) then you can just select the "emX macvtap" entry in virt-manager's network connection selection, and set it to passthrough mode.

I tried using the GUI and virsh to create the bridge. I ended up with this config:

That's all standard.

[root cbcvm network-scripts]# cat ifcfg-em4
[root cbcvm network-scripts]# cat ifcfg-www_br0
[root cbcvm network-scripts]# virsh iface-list
Name                 State      MAC Address
em1                  active     d4:ae:52:a1:53:a4
lo                   active     00:00:00:00:00:00
mon_br0              active     d4:ae:52:a1:53:a6
sbs_br0              active     d4:ae:52:a1:53:a5
www_br0              active     d4:ae:52:a1:53:a7

How to I make the expected vnet3 device become available to my VM?

You tell your guest to connect to whichever bridge device you want, then start the guest - a "vnetX" device will automatically be created (using some unused number). That device will attach to the bridge on the front end, and have a socket connecting to the guest's qemu on the backend. When the guest is shutdown, the vnetX device will be deleted, and a new one created the next time the guest is started.


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