CowboyRobot writes: At the recent 2013 Open Networking Summit, Google Distinguished Engineer Amin Vahdat presented "SDN@Google: Why and How", in which he described Google's "B4" SDN network, one of the few actual implementations of software-defined networking. Google has deployed sets of Network Controller Servers (NCSs) alongside the switches, which run an OpenFlow agent with a "thin level of control with all of the real smarts running on a set of controllers on an external server but still co-located." By using SDN, Google hopes to increase efficiency and reduce cost. Unlike computation and storage, which benefit from an economy of scale, Google's network is getting much more expensive each year. Link to Original Source
You can do this in a number of ways. IBM chose to do all of them.
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-- D. Taylor, Computer Science 350