- A
Application layer
Why wrong: The application layer consists of network services and applications that communicate with the controller via northbound APIs, but it does not directly make forwarding decisions.
- B
Control layer
The control layer (SDN controller) centralizes the control plane and programs switches via southbound APIs, determining how packets should be forwarded.
- C
Data plane
Why wrong: The data plane is distributed on switches and is responsible for forwarding packets based on flow tables installed by the controller. It does not make independent forwarding decisions in an SDN architecture.
- D
East-west interfaces
Why wrong: East-west interfaces are used for controller-to-controller communication in a distributed SDN environment, not for programming physical switches.
Quick Answer
The answer is the control layer. This is correct because in Software-Defined Networking (SDN), the control layer acts as the centralized brain that makes all forwarding decisions, then programs the data plane switches by communicating those decisions via southbound APIs like OpenFlow. This separation of the control plane from the data plane is the core principle of SDN, allowing for dynamic, centralized network management. On the CompTIA Network+ N10-009 exam, this concept tests your understanding of SDN architecture layers and their functions; a common trap is confusing the control layer with the application layer, which handles business logic, or the data plane, which only forwards packets based on pre-installed flow tables. To remember it, think of the control layer as the "controller" that tells switches where to send traffic, while southbound APIs are the "southbound instructions" that carry those orders down to the hardware.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network architect is designing a Software-Defined Networking (SDN) based network. Which of the following components is responsible for making centralized forwarding decisions and communicating those decisions to the physical switches using southbound APIs?
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
Control layer
In SDN architecture, the control layer (also known as the SDN controller) is the centralized entity responsible for making all forwarding decisions. It communicates these decisions to the physical switches via southbound APIs, such as OpenFlow, which program the flow tables in the data plane. This separation of the control plane from the data plane is the fundamental principle of SDN.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Application layer
Why it's wrong here
The application layer consists of network services and applications that communicate with the controller via northbound APIs, but it does not directly make forwarding decisions.
- ✓
Control layer
Why this is correct
The control layer (SDN controller) centralizes the control plane and programs switches via southbound APIs, determining how packets should be forwarded.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Data plane
Why it's wrong here
The data plane is distributed on switches and is responsible for forwarding packets based on flow tables installed by the controller. It does not make independent forwarding decisions in an SDN architecture.
- ✗
East-west interfaces
Why it's wrong here
East-west interfaces are used for controller-to-controller communication in a distributed SDN environment, not for programming physical switches.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the control layer with the data plane, thinking that the physical switches themselves make forwarding decisions, but in SDN the control layer is logically centralized and the switches are simple forwarding devices.
Detailed technical explanation
How to think about this question
Southbound APIs like OpenFlow (versions 1.0 through 1.5) use a secure TLS connection between the controller and the switch to install, modify, or delete flow entries. The controller can reactively install flows upon the first packet of a flow (punt) or proactively push all flows in advance. In a real-world scenario, a controller like OpenDaylight or ONOS can manage thousands of switches, and the southbound protocol must handle flow table overflow by using multiple tables or group tables for efficient forwarding.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Control layer — In SDN architecture, the control layer (also known as the SDN controller) is the centralized entity responsible for making all forwarding decisions. It communicates these decisions to the physical switches via southbound APIs, such as OpenFlow, which program the flow tables in the data plane. This separation of the control plane from the data plane is the fundamental principle of SDN.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This N10-009 practice question is part of Courseiva's free CompTIA certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the N10-009 exam.
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