What is the function of a controller in a software-defined network?

setting packet-handling policies. In a software-defined network, the controller is responsible for centralized control decisions and policy, including packet-handling instructions that are pushed toward the network devices. It does not perform ordinary data-plane packet forwarding; that remains the job of switches and routers. Hardware multicast replication and packet fragmentation are also not the controller’s defining function. Cisco CCNA 200-301 v1.1 Automation and Programmability expects candidates to understand the separation of the control plane and data plane. The corrected answer is setting packet-handling policies. The controller determines desired behavior and communicates it through southbound mechanisms, while infrastructure devices enforce that behavior in the forwarding plane.