The answer is that the --discovery-token-ca-cert-hash value is incorrect. This error occurs because the kubeadm join process uses the provided CA certificate hash to verify the identity of the API server during the TLS bootstrap handshake; when the hash doesn't match the actual hash of the control plane's CA certificate, the preflight check fails with the message about being unable to validate the API server's identity. On the Certified Kubernetes Administrator CKA exam, this scenario tests your understanding of secure node joining and the discovery-token-ca-cert-hash parameter, which is a common trap where candidates might mistakenly focus on token expiry or network issues instead of a simple typo in the hash. A reliable memory tip is to always generate the correct hash by running 'openssl x509 -pubkey -in /etc/kubernetes/pki/ca.crt | openssl rsa -pubin -outform der 2>/dev/null | openssl dgst -sha256 -hex | sed 's/^.* //'' on the control plane node, ensuring the hash matches exactly what you pass to the worker node.
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 new Kubernetes administrator runs 'kubeadm join --token <token> <control-plane-ip>:6443 --discovery-token-ca-cert-hash sha256:<hash>' on a worker node. The join fails with 'error execution phase preflight: couldn't validate the identity of the API Server'. What 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 --discovery-token-ca-cert-hash value is incorrect
The error 'couldn't validate the identity of the API Server' indicates that the CA certificate hash provided with --discovery-token-ca-cert-hash does not match the actual hash of the API server's CA certificate. This hash is used to verify the API server's identity during the TLS bootstrap process. An incorrect hash value will cause the preflight check to fail, as the worker node cannot confirm it is connecting to the legitimate control plane.
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 --discovery-token-ca-cert-hash value is incorrect
Why this is correct
The error message explicitly mentions 'couldn't validate the identity', which is a CA hash mismatch.
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 token has expired
Why it's wrong here
An expired token would produce a different error, such as 'token is invalid'.
✗
The kubelet is not running on the worker node
Why it's wrong here
A kubelet not running would cause a different error during the preflight check.
✗
The API server is not reachable on port 6443
Why it's wrong here
If the API server were unreachable, the error would be a connection timeout or refused.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CNCF often tests the distinction between token expiration and CA hash mismatch, where candidates confuse a token-related error with a TLS validation error, but the specific phrase 'couldn't validate the identity of the API Server' directly points to the CA certificate hash being incorrect.
Detailed technical explanation
How to think about this question
The --discovery-token-ca-cert-hash flag expects a SHA-256 hash of the root CA certificate used by the API server, computed as `openssl x509 -pubkey -in /etc/kubernetes/pki/ca.crt | openssl rsa -pubin -outform der 2>/dev/null | openssl dgst -sha256 -hex | sed 's/^.* //'`. This hash is part of the TLS bootstrap process defined in the Kubernetes cluster authentication flow, ensuring the worker node only joins a trusted control plane. In real-world scenarios, a common mistake is copying the hash from the wrong control plane node or using a hash from a different cluster, leading to this exact error.
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 --discovery-token-ca-cert-hash value is incorrect — The error 'couldn't validate the identity of the API Server' indicates that the CA certificate hash provided with --discovery-token-ca-cert-hash does not match the actual hash of the API server's CA certificate. This hash is used to verify the API server's identity during the TLS bootstrap process. An incorrect hash value will cause the preflight check to fail, as the worker node cannot confirm it is connecting to the legitimate control plane.
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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Question Discussion
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