Quick Answer
The answer is a mismatch in the encryption algorithm, such as AES256 versus 3DES. This is the most likely incorrect setting because Phase 1 negotiation relies on both peers agreeing on every parameter in the IKE proposal; when the debug command shows mismatched proposals, the encryption algorithm is the most common culprit due to its strict, exact-match requirement. On the Fortinet NSE 7 Advanced Security exam, this scenario tests your ability to interpret `diagnose vpn ike config` output and isolate the specific proposal element causing the failure, often appearing as a trap where candidates blame the pre-shared key or peer ID instead. Remember that Phase 1 is all about establishing a secure channel, and every detail—encryption, hash, Diffie-Hellman group, and lifetime—must match exactly, but the encryption algorithm is the first place to check when you see a proposal mismatch. A helpful memory tip: "Encryption first, then hash, then DH—if Phase 1 fails, check the cipher's path."
This NSE7 practice question tests your understanding of troubleshooting and diagnostics. 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 VPN tunnel shows 'phase1 negotiation failed' in the logs. The remote gateway is a third-party device. The debug command 'diagnose vpn ike config' shows mismatched proposals. Which setting is MOST likely incorrect on the FortiGate?
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.
The pre-shared key
Why wrong: Wrong PSK causes authentication failure, not proposal mismatch.
The local ID type
Why wrong: Local ID mismatch usually causes phase2 issues, not phase1.
The encryption algorithm (e.g., AES256 vs 3DES)
Mismatched encryption algorithms cause phase1 failure.
The DPD configuration
Why wrong: DPD is for keepalive, not negotiation.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The encryption algorithm (e.g., AES256 vs 3DES)
Phase 1 negotiation fails when proposals do not match. Common causes include encryption algorithm, hash, DH group, or lifetime mismatch. Option A is correct because the encryption algorithm is a key part of the proposal.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The pre-shared key
Why it's wrong here
Wrong PSK causes authentication failure, not proposal mismatch.
The local ID type
Why it's wrong here
Local ID mismatch usually causes phase2 issues, not phase1.
The encryption algorithm (e.g., AES256 vs 3DES)
Why this is correct
Mismatched encryption algorithms cause phase1 failure.
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 DPD configuration
Why it's wrong here
DPD is for keepalive, not negotiation.
Common exam traps
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
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the NSE7 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
Identify which NSE7 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.
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FAQ
Troubleshooting and Diagnostics — This question tests Troubleshooting and Diagnostics — Read the scenario before looking for a memorised answer..
The correct answer is: The encryption algorithm (e.g., AES256 vs 3DES) — Phase 1 negotiation fails when proposals do not match. Common causes include encryption algorithm, hash, DH group, or lifetime mismatch. Option A is correct because the encryption algorithm is a key part of the proposal.
Identify which NSE7 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.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 21, 2026
This NSE7 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 NSE7 exam.
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