The answer is the remote peer using AES256 instead of AES128. This is correct because an IPsec VPN Phase 1 failure with a 'no proposal chosen' error occurs when the encryption, authentication, Diffie-Hellman group, or lifetime parameters do not match between peers; IKEv1 main mode requires an exact match on all proposal attributes before proceeding to Phase 2. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of IPsec proposal negotiation and the common misconfiguration trap where one side uses a stronger cipher like AES256 while the other expects AES128, causing the error. A frequent memory tip is to remember that “Phase 1 is picky—every parameter must match exactly, or you get ‘no proposal chosen’.” For quick recall, think “AES128 vs AES256: one bit of difference, all the mismatch.”
NSE4 Authentication and VPN Practice Question
This NSE4 practice question tests your understanding of authentication and vpn. 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.
An administrator configures an IPsec VPN with IKEv1 main mode. The remote peer reports that Phase 1 fails with a 'no proposal chosen' error. The local Phase 1 settings include: encryption AES128, authentication SHA1, DH group 2, lifetime 86400. Which remote peer setting is MOST likely causing the mismatch?
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.
Why wrong: If local is main mode and remote is aggressive mode, the negotiation will fail. However, the error 'no proposal chosen' typically indicates parameter mismatch, not mode mismatch.
B
Remote peer uses AES256 instead of AES128
AES128 vs AES256 is a common mismatch. Both are valid but different.
C
Remote peer uses DH group 5 instead of group 2
Why wrong: DH group mismatch also causes failure, but the question expects the most common mistake.
D
Remote peer uses SHA256 instead of SHA1
Why wrong: SHA256 is different but if remote uses SHA256 and local SHA1, that would cause mismatch. However, the question asks for MOST likely, and the given settings include SHA1.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remote peer uses AES256 instead of AES128
Main mode requires both sides to have matching parameters. A mismatch in any parameter causes 'no proposal chosen' error. The remote peer likely has AES256 instead of AES128.
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.
✗
Remote peer uses aggressive mode
Why it's wrong here
If local is main mode and remote is aggressive mode, the negotiation will fail. However, the error 'no proposal chosen' typically indicates parameter mismatch, not mode mismatch.
✓
Remote peer uses AES256 instead of AES128
Why this is correct
AES128 vs AES256 is a common mismatch. Both are valid but different.
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.
✗
Remote peer uses DH group 5 instead of group 2
Why it's wrong here
DH group mismatch also causes failure, but the question expects the most common mistake.
✗
Remote peer uses SHA256 instead of SHA1
Why it's wrong here
SHA256 is different but if remote uses SHA256 and local SHA1, that would cause mismatch. However, the question asks for MOST likely, and the given settings include SHA1.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 NSE4 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.
Identify which NSE4 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Authentication and VPN — This question tests Authentication and VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Remote peer uses AES256 instead of AES128 — Main mode requires both sides to have matching parameters. A mismatch in any parameter causes 'no proposal chosen' error. The remote peer likely has AES256 instead of AES128.
What should I do if I get this NSE4 question wrong?
Identify which NSE4 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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