Which function normally performed by a traditional network device is moved to a software-defined controller?

In Software-Defined Networking (SDN), the control plane, responsible for making forwarding decisions, is decoupled from the data plane, which handles the actual packet forwarding. The SDN controller takes over the control plane functions, including building route tables and updating the forwarding tables (FIB/CAM tables) on network devices. This centralization allows for programmatic control and automation of the network. Encapsulation/decapsulation, NAT, and encryption/decryption are data plane functions that typically remain on the network devices themselves, as they involve processing individual packets at high speeds.