The answer is a mismatch in the Diffie-Hellman group and the encryption algorithm. When an IPsec VPN fails to establish and the Phase 1 status shows 'init' before resetting, the 'no acceptable proposal' message in the IKE debug output means the two peers cannot agree on a common set of security parameters during the negotiation process. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of IKE Phase 1 proposal matching, where both sides must share identical settings for encryption, authentication, and DH group; a common trap is assuming only the pre-shared key is wrong, but the debug clearly points to proposal mismatches. To remember this, think of Phase 1 as a handshake where both parties must speak the same language—if the DH group or cipher doesn’t match, the conversation ends before it starts.
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.
A FortiGate administrator is troubleshooting an IPsec VPN that fails to establish. The Phase 1 status shows 'init' and then resets. The administrator runs 'diagnose debug application ike -1' and sees the message 'no acceptable proposal'. Which TWO parameters are MOST likely mismatched?
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
✓
Encryption algorithm (e.g., AES256 vs AES128)
The 'no acceptable proposal' error in Phase 1 indicates that the local and remote peers cannot agree on a set of parameters. The encryption algorithm and Diffie-Hellman group are common mismatched parameters.
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.
✗
Pre-shared key
Why it's wrong here
Pre-shared key mismatch would show 'authentication failed', not 'no acceptable proposal'.
✗
Phase 2 local and remote networks
Why it's wrong here
Phase 2 networks are negotiated after Phase 1.
✗
IKE version (IKEv1 vs IKEv2)
Why it's wrong here
IKE version mismatch would cause a different error or no response.
✓
Encryption algorithm (e.g., AES256 vs AES128)
Why this is correct
Mismatched encryption algorithms cause proposal 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.
✓
Diffie-Hellman group (e.g., group 14 vs group 2)
Why this is correct
DH group must match to establish the shared secret.
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
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.
Trap categories for this question
Command / output trap
Pre-shared key mismatch would show 'authentication failed', not 'no acceptable proposal'.
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: Encryption algorithm (e.g., AES256 vs AES128) — The 'no acceptable proposal' error in Phase 1 indicates that the local and remote peers cannot agree on a set of parameters. The encryption algorithm and Diffie-Hellman group are common mismatched parameters.
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.
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 →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. An administrator is troubleshooting an IPsec VPN that fails to establish Phase 2. The Phase 1 is up. The administrator runs 'diagnose vpn ike log' and sees the message 'no matching phase2 proposal found'. What is the MOST likely cause?
hard
A.Pre-shared key mismatch
B.IKE version mismatch (IKEv1 vs IKEv2)
C.Phase 1 encryption algorithm mismatch
✓ D.Phase 2 proxy ID mismatch
Why D: Phase 2 uses the proxy IDs (local/remote subnets) to match proposals. If the remote peer expects different subnets, Phase 2 will fail.
Last reviewed: Jun 21, 2026
Question Discussion
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This NSE4 practice question is part of Courseiva's free Fortinet 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 NSE4 exam.
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