200-901 Network Fundamentals Practice Question
In an SDN architecture, which layer is responsible for making decisions about where traffic should be forwarded?
⚠ Common exam trap
Cisco often tests the distinction between the control plane and management plane, where candidates mistakenly think that configuration (management plane) is the same as making forwarding decisions, but the control plane is the one that dynamically determines traffic paths.
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
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Control plane
In SDN architecture, the control plane is responsible for making forwarding decisions by maintaining the network topology and computing paths for traffic flows. It communicates these decisions to the data plane via southbound protocols like OpenFlow or NETCONF, ensuring that packets are forwarded according to the desired policies.
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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Data plane
Why it's wrong here
The data plane forwards packets according to rules the control plane installs; it does not decide paths. It is tempting because forwarding and routing sound alike, but path computation belongs to the control plane, which programs the data plane's flow tables.
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Application plane
Why it's wrong here
The application plane hosts business and network programs that express intent through northbound APIs; it does not compute forwarding decisions. It is tempting because applications request behaviour, but the control plane translates that intent into the forwarding rules installed on the data plane.
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Management plane
Why it's wrong here
The management plane configures and monitors devices; forwarding decisions belong to the control plane, which computes routes and populates the data plane. Management-plane tooling (SNMP, NETCONF, syslog) is the right answer when the question asks how an SDN controller is administered or how policy is provisioned, not where forwarding logic resides.
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Control plane
Why this is correct
The control plane holds the centralised logic that computes forwarding decisions, programming them down to the data plane via southbound APIs. The data plane merely executes those instructions, which is the SDN separation of concerns.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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