Good post on SDN from Sandip Lahiri (thank you)
Software Defined Networking (SDN) is an emerging network architecture that reduces cost while increasing quality and service agility over existing networks. Cost includes both capital and operational expenditures. Quality spans network security, robustness, manageability, mobility and flexibility. Service agility allows rapid introduction of new services on the network. (“Why SDN?” offers a good overview.)
A traditional networking device, for example a switch or a router, lumps network control functions together with a data-forwarding mechanism. These components are complex and proprietary, resulting in major shortcomings with the current networking technology. SDN separates these functions with open interfaces. It simplifies networking and lets us innovate.
Read on here