The answer is a Service targetPort mismatch, which is the most likely cause when a ClusterIP Service is unreachable from within the cluster despite the pods being running and responsive. This occurs because the Service’s `targetPort` must exactly match the `containerPort` defined in the pod’s container spec; if they differ, traffic is forwarded to the wrong port on the pod, causing the connection to fail even though the pods themselves are healthy. On the CKA exam, this scenario tests your understanding of how Kubernetes Service networking routes traffic—specifically the distinction between the Service’s `port` (the listening port) and `targetPort` (the actual destination on the pod). A common trap is assuming that as long as pods are running, any port will work, but the exam expects you to verify the `targetPort` aligns with the container’s listening port. Memory tip: think of the Service as a receptionist—the `port` is the front desk number, but the `targetPort` is the specific office extension the call must reach.
CKA Practice Question: Cluster Architecture, Installation and Configuration
This CKA practice question tests your understanding of cluster architecture, installation and configuration. 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.
A user reports that they cannot access a Service of type ClusterIP from within the cluster. The Service selects pods that are running and responding. Which of the following is the MOST likely cause?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Service's targetPort does not match the container's containerPort
Option C is correct because the Service's `targetPort` must match the `containerPort` defined in the pod's container spec. Even if the pods are running and responding, if the `targetPort` points to a different port than the one the container is listening on, traffic will be forwarded to the wrong port and the connection will fail. The `port` field on the Service is the port the Service itself listens on, while `targetPort` is the port on the pod where traffic is actually sent.
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 Service's port does not match the container's containerPort
Why it's wrong here
The Service's port is the port exposed by the Service, not necessarily the container port. The targetPort must match the container port.
✗
The Service type is NodePort instead of ClusterIP
Why it's wrong here
NodePort also allows internal access; type does not prevent internal access.
✓
The Service's targetPort does not match the container's containerPort
Why this is correct
If targetPort is not set correctly, traffic will not reach the container.
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.
✗
The Service selector does not match any pod labels
Why it's wrong here
The scenario says the Service selects pods that are running and responding, so selector is fine.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse the Service's `port` (the cluster-facing port) with the `targetPort` (the pod-facing port), and incorrectly assume the `port` must match the container's `containerPort`, leading them to choose Option A instead of C.
Trap categories for this question
Scenario analysis trap
The scenario says the Service selects pods that are running and responding, so selector is fine.
Detailed technical explanation
How to think about this question
When a Service forwards traffic, it uses iptables or IPVS rules to DNAT the destination IP:port to one of the selected pod's IP and the `targetPort`. If `targetPort` is omitted, it defaults to the value of `port`, but if explicitly set to a different value, it must match the container's `containerPort`. A common real-world scenario is when a container listens on port 8080 but the Service's `targetPort` is set to 80, causing all traffic to be dropped because no process is listening on port 80 inside the pod.
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 CKA 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CKA question in full detail.
Cluster Architecture, Installation and Configuration — This question tests Cluster Architecture, Installation and Configuration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Service's targetPort does not match the container's containerPort — Option C is correct because the Service's `targetPort` must match the `containerPort` defined in the pod's container spec. Even if the pods are running and responding, if the `targetPort` points to a different port than the one the container is listening on, traffic will be forwarded to the wrong port and the connection will fail. The `port` field on the Service is the port the Service itself listens on, while `targetPort` is the port on the pod where traffic is actually sent.
What should I do if I get this CKA 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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