200-901 Network Fundamentals Practice Question
In Software-Defined Networking (SDN), which interface is used for communication between the controller and the network devices (e.g., switches) to forward traffic?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Southbound API
The southbound API (e.g., OpenFlow, NETCONF) is used to communicate between the SDN controller and the data plane devices.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
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Northbound API
Why it's wrong here
Northbound APIs expose controller capabilities upward to applications and orchestration systems, not downward to switches. It is tempting because northbound interfaces are central to SDN programmability, but they carry application intent rather than the forwarding rules delivered via the Southbound API.
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Control plane
Why it's wrong here
The control plane is a functional layer that computes forwarding decisions, not an interface carrying them to devices. It is tempting because the controller resides there, but the question asks which interface transports rules to switches, which is the Southbound API.
- ✓
Southbound API
Why this is correct
The southbound API connects the SDN controller to underlying switches, letting it push flow rules and forwarding instructions directly to the data plane. This satisfies the stem's requirement for controller-to-device communication, whereas northbound interfaces serve applications and management layers instead.
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East-West API
Why it's wrong here
East-West APIs connect controllers to each other within the control plane, not to switches. It is tempting because east-west traffic describes peer communication, but forwarding instructions to data-plane devices travel over the Southbound API, which this scenario requires.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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