In Windows 2008/R2 Hyper-V, a virtual network was the term that was used to describe the switch that connected a physical network card to the port of a VM’s virtual network adapter. That has changed in Windows Server 8; it is now referred to as a virtual switch, or to be more precise, the extensible virtual switch.
Why extensible? Microsoft has made it possible for 3rd party software developers to plug into the switch and add more functionality. One such example is Cisco, who have developed a solution. To put it simply, using extensions, you can extend your Cisco network into Hyper-V networking. I heard about it on Twitter, and then I heard that Cisco had a booth at Build Windows so I went to talk to them, and got a demo.
Wait a moment: I have had the next question twice when working with senior Cisco network engineers. I asked Cisco the question, and their eyes rolled; they’d heard this question non-stop since opening the booth 
How will virtual switches, to be precise, Cisco virtual switches deal with spanning tree? The answer was that “they will break the loop” so there should be no problem.
The core advantage for customers that do this is that they can use a single management solution and skill set to manage all of networking. In the demo, I was shown how everything about the virtual switch in the Cisco command line console was very similar, if not identical, to managing a physical switch.
Additionally you get the power and configurability of Cisco networking. For example, in a GUI, you could create Port policies to dictate:
- What a port could talk to
- What protocol it could use
- Etc
You assigned a policy to the port and suddenly it was filtering – but this was all done using Cisco tools that network admins already know. Another integration was VLAN support for ports.
Pretty powerful stuff!