200-901 Network Fundamentals Practice Question
In the context of SDN, which API is used between the SDN controller and the network devices to configure forwarding behavior?
⚠ Common exam trap
200-901 often tests the northbound vs. southbound direction confusion — candidates must remember that southbound points down to devices, northbound points up to applications.
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 is the interface between the SDN controller and the underlying network devices (switches, routers), used to program forwarding tables and configure behavior. Protocols like OpenFlow, NETCONF, and P4 are typical southbound interfaces. The controller translates northbound application intent into southbound device instructions.
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 sit between the SDN controller and applications or orchestration systems, exposing network abstractions upward. The controller-to-device forwarding configuration happens over southbound APIs such as OpenFlow. Northbound interfaces would be the right choice when an application needs to request network services from the controller.
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Eastbound API
Why it's wrong here
Eastbound APIs connect multiple SDN controllers across domains, enabling controller-to-controller communication for federated or hierarchical control. They do not configure forwarding behaviour on individual network devices. Eastbound interfaces would be correct when coordinating separate controller domains rather than programming data-plane devices.
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Southbound API
Why this is correct
The southbound API sits below the controller, translating its forwarding decisions into device-level configuration. OpenFlow is the canonical example, programming match-action flow tables on switches. This satisfies the stem's requirement for the interface used to configure forwarding behaviour on network devices.
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REST API
Why it's wrong here
REST is a protocol style, not an SDN interface category; it can carry northbound or eastbound calls but does not itself define controller-to-device forwarding configuration. Southbound APIs such as OpenFlow or NETCONF serve that role. REST would be the right answer when describing how applications invoke controller APIs.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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