In SDN architecture, which component centralizes network control?

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

Answer: (A)

In SDN, the control plane is separated from the data plane, and the central brain of the network is the software controller. This controller maintains a global view of the network topology and policies, makes routing and forwarding decisions, and programs the forwarding devices with the appropriate rules. The forwarding devices themselves act only on those instructions to move traffic, which is why they’re considered data-plane elements. The firmware on switches is just device software for operation and doesn’t provide a network-wide control decision-making capability. Physical routers, in a traditional setup, also carry out routing decisions locally rather than being centrally managed. The controller’s centralized role is what enables global optimization, easier policy enforcement, and rapid network changes.