Re: vnet device not appearing

Laine Stump <laine@xxxxxxxxx> · Wed, 28 May 2014 16:56:06 +0300



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

    
      Hello
        

        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
            DEVICE=em4
            HWADDR=d4:ae:52:a1:53:a7
            _ONBOOT_=yes
            BRIDGE=www_br0
            [root@cbcvm
                network-scripts]# cat ifcfg-www_br0
            DEVICE=www_br0
            _ONBOOT_=yes
            TYPE=Bridge
            STP=on
            DELAY=0
          
          
              [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.

    
          Thanks!
          

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