200-901 Network Fundamentals Practice Question
In an SDN architecture, which API is used by the controller to communicate with network devices to install forwarding rules?
⚠ Common exam trap
Cisco often tests the distinction between northbound and southbound APIs; the trap here is confusing the REST API (commonly northbound) with the southbound API that directly programs device forwarding tables.
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
In SDN, the southbound API is the interface between the controller and the network devices (switches, routers). It allows the controller to install forwarding rules, such as flow entries in OpenFlow switches, enabling centralized control of the data plane.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Southbound API
Why this is correct
The southbound API connects the SDN controller downward to forwarding devices, programming flow tables and installing forwarding rules. This satisfies the requirement to communicate with network devices, whereas northbound APIs face applications and management planes.
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REST API
Why it's wrong here
REST is a generic API style, not the SDN controller-to-device interface; that role belongs to the southbound API, which may use protocols such as OpenFlow, NETCONF or PCEP. It is tempting because REST is widely used for controller northbound interfaces and management, where it is the correct choice.
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East-West API
Why it's wrong here
East-West APIs connect peer controllers in a distributed control plane, exchanging topology and state between them; they never program forwarding rules on data-plane devices. It is tempting because inter-controller communication is a genuine SDN interface, correct when controllers must synchronise across domains.
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Northbound API
Why it's wrong here
The northbound API faces applications and orchestration systems above the controller, exposing network abstractions; it does not install forwarding rules on devices. It is tempting because it is the API most developers interact with, and it is correct when an application requests services from the controller.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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