Hyper-V and networking

For those who have worked with hosted virtualisation (Microsoft Virtual PC and Virtual Server, VMware Workstation and Server, Parallels Desktop, etc.) and haven’t experienced hypervisor-based virtualisation, Microsoft Hyper-V is fundamentally different in a number of ways. Architecturally, it’s not dissimilar to the Xen hypervisor (in fact, there are a lot of similarities between the two) and Xen’s domain 0 is analogous to the parent partition in Hyper-V (effectively, when the Hyper-V role is added to a Windows Server 2008 computer, the hypervisor is “slid” underneath the existing Windows installation and that becomes the parent partition). Subsequent virtual machines running on Hyper-V are known as child partitions.

In this approach, a new virtual switch (vswitch) is created and the physical network adapter (pNIC) is unbound from all clients, services and protocols, except the Microsoft Virtual Network Switch Protocol. The virtual network adapters (vNICs) in the parent and child partitions connect to the vswitch. Further vswitches may be created for internal communications, or bound to additional pNICs; however only one vswitch can be bound to a particular pNIC at any one time. Virtual machines can have multiple vNICs connected to multiple vswitches. Ben Armstrong has a good explanation of Hyper-V networking (with pictures) on his blog.

One exception relates to the connection of virtual machines to wireless network adapters (not a common server scenario, but nevertheless useful when Windows Server 2008 is running on a notebook PC). The workaround is to use Internet connection sharing (ICS) on the wireless pNIC and to connect that to a vswitch configured for internal networking in Hyper-V. Effectively, the ICS connection becomes a DHCP server for the 192.168.0.0/24 network, presented via the internal vswitch and I’m pleased to find that the same principle can be applied to mobile data cards. Interestingly, Hyper-V seems quite happy to bind directly to a Bluetooth connection.

Hyper-V network connection example

Using this approach, on my system, the various network adapters are as follows:

  • Dial-up adapters, including an HSDPA/HSUPA modem which I have shared to allow a VMs to connect to mobile networks in place of wired Ethernet.
  • Local Area Connection – the pNIC in my notebook PC, bound only to to the Microsoft Virtual Network Switch Protocol.
    Wireless Network Connection – the WiFi adapter in my notebook PC (if there was WiFi connectivity where I am today then this could have been shared instead of the data card.
  • Local Area Connection 3 – the Bluetooth adapter in my notebook PC.
  • Local Area Connection 4 – the external vswitch in my Hyper-V installation, connected to the external network via the pNIC.
  • Local Area Connection 5 – another vswitch in my Hyper-V installation, operating as an internal network, but connected using the method above to the shared HSDPA/HSUPA modem.

This gives me plenty of flexibility for connectivity and has the useful side-effect of allowing me to circumvent the port security which I suspect is the cause of my frequent disconnections at work because the physical switches are configured to block any device presenting multiple MAC addresses for the same port.

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