In an SDN architecture, the SDN controller sits between the network infrastructure and the applications.

There are two main API directions:

Northbound APIs: between the SDN controller and applications

Southbound APIs: between the SDN controller and network devices

Applications communicate with the controller using northbound APIs, and these are commonly implemented as REST APIs.

REST API is the common method used by applications to interact with the SDN controller.

It allows software and orchestration platforms to request network information or make policy changes through HTTP-based calls.

B. OpenFlow: used between the controller and network devices, not applications.

C. Southbound API: this refers to controller-to-device communication, not controller-to-application communication.

D. NETCONF: used for device configuration and management, typically between controllers/management platforms and devices, not primarily for app-to-controller communication.

Why A is correct:Why the others are incorrect:CCNA takeaway:If the question asks about applications talking to the SDN controller, think:

Northbound API → usually REST API