Which API is used in controller-based architectures to interact with edge devices?

In controller-based networking, the southbound API is the interface between the controller and the network devices it manages. The controller uses southbound communication to discover devices, collect operational data, push configuration, and influence forwarding behavior. Cisco CCNA 200-301 v1.1 separates this from northbound APIs, which are used by applications, orchestration tools, or business systems to communicate with the controller. The answer cannot be overlay or underlay because those terms describe logical and physical network transport designs, not API direction. Northbound is also wrong because it faces upward toward applications, not downward toward routers, switches, and access points. Common southbound technologies include NETCONF, RESTCONF, gNMI, and in SDN discussions, OpenFlow. The safest exam model is directional: northbound equals application-to-controller, while southbound equals controller-to-device. Since edge devices are the managed infrastructure, the API used to interact with them is southbound.