Which function generally performed by a traditional network device is replaced by a software- defined controller?

In traditional networking, individual routers and switches autonomously build their own route and forwarding tables using distributed control plane protocols. A software-defined controller centralizes the control plane, taking over the responsibility of computing and distributing forwarding rules (flow entries) to network devices, thereby replacing the device's traditional role in building and updating these tables.

Encapsulation and decapsulation of packets in a data-link frame is a data plane function performed by network interface cards (NICs) and switches, which remains largely distributed in SDN.

Changing source or destination addresses during NAT operations is a data plane function performed by routers or firewalls, which SDN controllers can orchestrate but do not directly execute.

Encryption and decryption for VPN link processing is a data plane security function typically performed by dedicated hardware or software on network devices, not a control plane function replaced by an SDN controller.