The correct answer is that the IKE phase 1 parameters (encryption, hash, DH group) do not match. This is because an "IKE authentication failed" error in IPsec specifically indicates a failure during phase 1, where the two peers must agree on a common set of security parameters before any authentication can occur. Even when the pre-shared keys are verified as correct, a mismatch in the encryption algorithm (like AES versus 3DES), the hash function (SHA-1 versus SHA-256), or the Diffie-Hellman group (group 2 versus group 14) will prevent the IKE Security Association from being established, causing the authentication step to never complete. On the ISC2 Certified in Cybersecurity CC exam, this question tests your understanding that "authentication failed" in IPsec logs does not always mean a wrong password—it often points to a configuration mismatch in the phase 1 proposal. A common trap is assuming the pre-shared key is the only cause, but the exam wants you to recognize that parameter negotiation happens first. Memory tip: think of IKE phase 1 as a "handshake before the handshake"—if the grip (parameters) doesn't match, the handshake (authentication) never starts.
ISC2 CC Network Security Practice Question
This CC practice question tests your understanding of network security. 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 company's remote access VPN uses IPsec with pre-shared keys. Employees report that they cannot connect from home. The VPN server logs show 'IKE authentication failed.' The help desk confirms the pre-shared keys are correct. 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 IKE phase 1 parameters (encryption, hash, DH group) do not match
Option D is correct because 'IKE authentication failed' specifically indicates a failure during IKE phase 1, where the peers negotiate security parameters. Even if the pre-shared key is correct, mismatched encryption (e.g., AES vs. 3DES), hash (e.g., SHA-1 vs. SHA-256), or Diffie-Hellman group (e.g., group 2 vs. group 14) will cause the IKE SA to fail, preventing phase 1 authentication from completing.
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 client is behind a NAT device that blocks IPsec traffic
Why it's wrong here
NAT traversal (NAT-T) is common and would not cause 'IKE authentication failed' if configured correctly.
✗
The VPN server is not responding to IKE requests
Why it's wrong here
If unresponsive, the error would be timeout, not authentication failure.
✗
The client's certificate has expired
Why it's wrong here
Using pre-shared keys, not certificates.
✓
The IKE phase 1 parameters (encryption, hash, DH group) do not match
Why this is correct
Mismatched parameters cause authentication failure despite correct keys.
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
ISC2 often tests the distinction between 'IKE authentication failed' (which points to phase 1 parameter mismatch or PSK error) and 'no response' (which points to firewall/NAT blocking), trapping candidates who assume NAT is always the culprit without reading the log message carefully.
Detailed technical explanation
How to think about this question
IKE phase 1 (main mode or aggressive mode) involves two message exchanges: the first pair negotiates the protection suite (encryption, hash, DH group), and the second pair performs Diffie-Hellman key exchange and authentication. If the protection suite proposals do not match, the responder sends a 'no proposal chosen' or 'authentication failed' notification, as the IKE SA cannot be established. In Cisco IOS, the command 'crypto isakmp policy' defines these parameters, and mismatched DH groups (e.g., group 2 vs. group 5) are a common misconfiguration in remote access VPNs.
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 developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CC question in full detail.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The IKE phase 1 parameters (encryption, hash, DH group) do not match — Option D is correct because 'IKE authentication failed' specifically indicates a failure during IKE phase 1, where the peers negotiate security parameters. Even if the pre-shared key is correct, mismatched encryption (e.g., AES vs. 3DES), hash (e.g., SHA-1 vs. SHA-256), or Diffie-Hellman group (e.g., group 2 vs. group 14) will cause the IKE SA to fail, preventing phase 1 authentication from completing.
What should I do if I get this CC 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This CC practice question is part of Courseiva's free ISC2 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 CC exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.