Software Defined Networking is becoming harder to define.   SDN used to be primarily associated with network communications service providers.  However, with the advent of Multi-Cloud Service Delivery and the need to optimize service performance across multiple service domains, the need for SDN optimization extends into both the intra-cloud and inter-cloud network fabric.

Windows Server 2012 and Hyper-V bring strong enhancements in network virtualization.  The Hyper-V Virtual Switch is a software-based layer-2 network switch that is available in Hyper-V Manager when installed in the Hyper-V server role. The switch includes programmatically managed and extensible capabilities to connect virtual machines to both virtual networks and the physical network. In addition, Hyper-V Virtual Switch provides policy enforcement for security, isolation, and service levels.  The Hyper-V Virtual Switch in Windows Server® 2012 introduces several new features and enhanced capabilities for tenant isolation, traffic shaping, protection against malicious virtual machines, and simplified troubleshooting.

For a good overview of the Hyper-V Virtual Switch see this recently updated article: 

http://technet.microsoft.com/en-us/library/hh831823.aspx

For analysis and a video about how network virtualization using the extension provided by Windows Server 2012 Hyper-V and NECs ProgrammableFlow and OpenFlow Enabled Software Defined Network will dramatically benefit cloud enterprises see the following links:

http://technet.microsoft.com/en-us/video/nec-programmable-flow-sdn-and-hyper-v-transform-private-clouds.aspx

http://www.sdncentral.com/sdn-blog/game-changer-openflow-on-hyper-v-finally-arrives-with-necs-new-pf1000/2013/01/