200-901 Network Fundamentals Practice Question
In Software-Defined Networking (SDN), the control plane is separated from the data plane. Which of the following best describes the function of the southbound API?
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
✓
Interface between the controller and network devices
Southbound API is used by the SDN controller to communicate with network devices (e.g., switches, routers) to enforce forwarding rules.
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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Interface between applications and the controller
Why it's wrong here
The southbound API connects the SDN controller to the data-plane forwarding devices, carrying flow rules and statistics. An application-to-controller interface is the northbound API, which suits programmatic network automation and orchestration requests rather than device-level programming.
- ✓
Interface between the controller and network devices
Why this is correct
The southbound API connects the SDN controller downward to the physical and virtual network devices, carrying forwarding instructions such as OpenFlow flow-table entries. This directly satisfies the stem's separation of control and data planes, since the controller programmes devices through this interface.
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Communication between two controllers
Why it's wrong here
The southbound API carries instructions from a controller down to data-plane devices; controller-to-controller communication uses east-west APIs. East-west interfaces suit distributed or hierarchical controller clusters that must share topology and state across domains, not programming individual forwarding devices.
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Interface between the control plane and management plane
Why it's wrong here
The southbound API links the controller to data-plane forwarding devices, not to a management plane. Management-plane interfaces handle device configuration, monitoring, and fault management, so they suit operational administration of network elements rather than controller-to-switch flow programming.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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