Which protocol is commonly used as a southbound protocol between a software-defined network controller and network devices?

• A. OpenFlow
• B. SNMP
• C. Protobuf
• D. SSH

Answer: (A)

In SDN, the controller must program the forwarding behavior of switches, which requires a dedicated channel for control messages to the data plane. OpenFlow is the prototypical southbound protocol for this purpose. It defines the specific messages the controller sends to switches to install, modify, or remove flow entries, request switch state, and coordinate actions like where to forward packets. This standardized, centralized control of how traffic is handled across the network is what enables the SDN paradigm.

SNMP focuses on monitoring and managing devices rather than live flow-table programming, so it doesn’t provide the necessary control primitives. Protobuf is a data-serialization format used by some APIs, not a protocol designed for controlling switches. SSH is for secure remote login and execution, not for directing how switches forward traffic.