Software Defined Networking (SDN) is an architecture that separates the control plane, which decides where traffic should go, from the data plane, which actually forwards it. In a traditional switch or router both live inside the same box. SDN lifts the decision making into a central software controller, so network behavior can be programmed and changed on the fly without reconfiguring hardware device by device.
For security this is powerful. When a host is found compromised, an operator can push a policy change from the controller and isolate that segment, block the traffic, or divert it to a honeypot across the whole environment in seconds, work that would take hours of manual edits on conventional gear. The flip side is that the controller becomes a concentrated target. Whoever controls it controls the network, so hardening, authenticating, and closely monitoring that controller is not optional.
How is SDN tested on the CISSP?
SDN appears in Domain 4 (Communication and Network Security). Understand how centralizing control changes the security model, the risk it concentrates, and the granular enforcement it enables. Review Domain 4: Communication and Network Security.