- A
The backend pods are not running on the nodes because the cluster has insufficient resources.
Why wrong: The pods are running and ready, so resource issues are not the cause.
- B
The frontend pod is using the wrong service name; it should be 'backend-service.default.svc.cluster.local'.
Why wrong: Using the short name 'backend-service' works within the same namespace, so this is not the issue.
- C
The backend service's port field is set to 8080, but the targetPort is not specified, so it defaults to the port field value, which is correct.
Why wrong: If targetPort is not specified, it defaults to the port value, which matches containerPort, so this would work.
- D
The backend deployment's containerPort is set to 8081, but the service's targetPort is 8080, causing a mismatch.
If containerPort does not match targetPort, the service sends traffic to a port on the pod that is not listening, resulting in connection refused.
CKAD Application Deployment Practice Question
This CKAD practice question tests your understanding of application deployment. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
You are deploying a microservice application on a Kubernetes cluster. The application consists of a frontend service (type: ClusterIP) and a backend service (type: ClusterIP). The frontend needs to communicate with the backend via DNS name 'backend-service'. The backend service runs three replicas, each listening on port 8080. You have created the backend service and deployment, and verified that pods are running. However, when you deploy the frontend pod and attempt to curl http://backend-service:8080 from within the frontend, you get 'connection refused'. The frontend pod is in the same namespace as the backend. You check the backend pods and they are all running and ready, and the endpoints object shows the correct pod IPs. You suspect a misconfiguration in the service definition. Which of the following is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The backend deployment's containerPort is set to 8081, but the service's targetPort is 8080, causing a mismatch.
Option D is correct because the backend service's `targetPort` defaults to the value of `port` (8080) when not explicitly set, but the backend deployment's `containerPort` is 8081. This mismatch means the service sends traffic to port 8080 on the pods, where no process is listening, resulting in a 'connection refused' error. The pods are running and ready, but the readiness probe (if any) might still pass if it checks a different path, while the actual application port is misaligned.
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.
- ✗
The backend pods are not running on the nodes because the cluster has insufficient resources.
Why it's wrong here
The pods are running and ready, so resource issues are not the cause.
- ✗
The frontend pod is using the wrong service name; it should be 'backend-service.default.svc.cluster.local'.
Why it's wrong here
Using the short name 'backend-service' works within the same namespace, so this is not the issue.
- ✗
The backend service's port field is set to 8080, but the targetPort is not specified, so it defaults to the port field value, which is correct.
Why it's wrong here
If targetPort is not specified, it defaults to the port value, which matches containerPort, so this would work.
- ✓
The backend deployment's containerPort is set to 8081, but the service's targetPort is 8080, causing a mismatch.
Why this is correct
If containerPort does not match targetPort, the service sends traffic to a port on the pod that is not listening, resulting in connection refused.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume the `port` and `targetPort` are always identical by default and overlook checking the actual container listening port, leading them to incorrectly suspect DNS or resource issues.
Detailed technical explanation
How to think about this question
In Kubernetes, the `targetPort` field in a Service definition specifies the port on the pod to which traffic should be forwarded. If omitted, it defaults to the `port` field value. However, if the container's `containerPort` (or the actual process listening inside the container) differs from this default, the service will route traffic to a closed port, causing a TCP RST and 'connection refused'. This is a common misconfiguration when developers assume `port` and `targetPort` are always the same, especially when the container runs on a non-standard port like 8081 while the service advertises 8080.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this CKAD question test?
Application Deployment — This question tests Application Deployment — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The backend deployment's containerPort is set to 8081, but the service's targetPort is 8080, causing a mismatch. — Option D is correct because the backend service's `targetPort` defaults to the value of `port` (8080) when not explicitly set, but the backend deployment's `containerPort` is 8081. This mismatch means the service sends traffic to port 8080 on the pods, where no process is listening, resulting in a 'connection refused' error. The pods are running and ready, but the readiness probe (if any) might still pass if it checks a different path, while the actual application port is misaligned.
What should I do if I get this CKAD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 CKAD practice question is part of Courseiva's free CNCF 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 CKAD exam.
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