In Software-Defined Networking, how are control plane and data plane separated?

• A. A software controller
• B. The firmware on switches
• C. The forwarding devices
• D. Physical routers only

Answer: (A)

The main idea being tested is the architectural split between decision-making and packet forwarding in Software-Defined Networking. In SDN, the control plane—the brain that makes routing and policy decisions—is centralized in a software controller. This controller has a global view of the network, runs the network logic, and programs how traffic should be handled across the network. The data plane, implemented by the forwarding devices (switches/routers), is responsible only for actually moving packets according to the rules it has been given by the controller. This separation enables centralized management, easy updates, and dynamic reconfiguration.

Why this fits best: using a software controller to host the control plane aligns with the SDN model of decoupling decision-making from the devices that forward traffic. The forwarding devices remain lightweight, functioning as data-plane elements that execute the controller’s instructions.

Why the other ideas don’t fit: embedding control logic in the switches’ firmware keeps the control plane inside each device, undoing the separation. The forwarding devices are the data plane and carry out the actual forwarding, not the control logic. Limiting to physical routers misses the broader SDN concept, which can involve virtual switches and software-defined components beyond just traditional hardware routers.