Re: Need review of new vswitch concept info for technical accuracy

Stefan Berger <stefanb@xxxxxxxxxxxxxxxxxx> · Tue, 19 Oct 2010 11:05:55 -0400



    On 10/16/2010 03:01 PM, Justin Clift wrote:
    Hi
      all,
      

      Working on the "Foundation Concepts" for virtual switches at the
      moment.
      

      Does anyone have time/inclination to check over the concept
      graphics
      

      thus far, for technical accuracy?
      

      Especially the "Routing mode" and "Non-routing" mode ones, as I'm
      not
      

      sure if that's the right terminology for them:
      

        http://wiki.libvirt.org/page/VirtualNetworking#Routing_mode
      

        http://wiki.libvirt.org/page/VirtualNetworking#Non-routing_mode
      

      General feedback, thoughts, and suggestions are welcome too of
      course. :)
      

    In case of the NAT I would mention that it is setting up
    Masquerading (rather than SNAT / DNAT) which basically only allows
    the VM to establish connections to the outside but not the other way
    around.

    
    From the page:

    
     Routed mode 
    Also commonly referred to as bridging. In this mode, the
      virtual switch is connected to the physical host LAN, passing
      guest network traffic back and forth without using NAT. In this
      mode, computers external to the host server directly address and
      communicate with guest virtual machines.

    
      Routing and bridging are different, the one working on l3, the
      other on l2. So I would not say that it is 'commonly referred to
      as bridging', since this is mixing different concepts. 

    
    I suppose you describe the configuration where eth0 is plugged
      into the bridge virbr0. If yes, I would title the section as
      'bridging' mode. Typically in this case the VMs pick up an address
      in the physical subnet from an infrastructure DHCP server and they
      can communicate towards the outside or be contacted from anywhere
      from the outside (assuming routable addresses). The addresses you
      are showing in the example picture 10.10.10.100 and 10.10.10.200
      are typically 'private', thus will only be routed in the local
      network, but that's ok.

    
    However, there is this other mode libvirt is supporting where the
      VM's interfaces are plugged in virbr0, thus they do bridging when
      one VM communicates with another VM. However, the bridge can be
      given an IP address and when a VM wants to talk to another host or
      VM in the network (beyond the VM-hosting host), the packets
      'escape' the bridge, are routed on the Linux host towards eth0 and
      then delivered to the other host in the local subnet. Now that is
      a mix between routing and bridging.

    
       Stefan

    
      Regards and best wishes,
      

      Justin Clift
      

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