The answer is encryption algorithm mismatch, authentication mismatch, or Diffie-Hellman group mismatch. These three settings define the Phase 1 proposal, and when the local FortiGate and the remote peer do not share at least one common combination of these parameters, the IKE negotiation fails with the “no proposal chosen” error. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this question tests your ability to distinguish Phase 1 proposal mismatches from other configuration errors; a common trap is selecting “lifetime mismatch,” which actually causes a different IKE error (like “responder lifetime mismatch”) rather than a proposal rejection. To remember the three correct causes, use the mnemonic “EAD” — Encryption, Authentication, and Diffie-Hellman group — since these are the core components of any IKE Phase 1 proposal that must match exactly for the tunnel to establish.
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 admin is troubleshooting an IPsec VPN that fails to establish. The output of 'diagnose debug application ike -1' shows: 'IKE: No proposal chosen from x.x.x.x'. The admin checks the Phase1 configuration. Which of the following mismatches could cause this error? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Diffie-Hellman group mismatch (e.g., group 2 vs group 14)
The error 'no proposal chosen' indicates that the local and remote IKE peers have no common Phase1 proposal. This can be caused by mismatched encryption, authentication, or Diffie-Hellman group. Lifetime mismatch usually results in a different error.
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.
✓
Diffie-Hellman group mismatch (e.g., group 2 vs group 14)
Why this is correct
DH group is also part of the proposal.
Related concept
Read the scenario before looking for a memorised answer.
✗
Pre-shared key mismatch
Why it's wrong here
PSK mismatch causes authentication failure, not proposal mismatch.
✗
Lifetime mismatch (e.g., 86400 vs 3600)
Why it's wrong here
Lifetime mismatch typically results in negotiation succeeding but rekey issues.
✓
Authentication method mismatch (e.g., SHA1 vs SHA256)
Why this is correct
Hash algorithm is part of Phase1 proposal.
Related concept
Read the scenario before looking for a memorised answer.
✓
Encryption algorithm mismatch (e.g., AES128 vs AES256)
Why this is correct
The encryption algorithm is part of the proposal; mismatch causes no common proposal.
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.
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: Diffie-Hellman group mismatch (e.g., group 2 vs group 14) — The error 'no proposal chosen' indicates that the local and remote IKE peers have no common Phase1 proposal. This can be caused by mismatched encryption, authentication, or Diffie-Hellman group. Lifetime mismatch usually results in a different error.
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.
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. A FortiGate admin is troubleshooting an IPsec VPN tunnel that fails to establish. The remote site uses aggressive mode. The local FortiGate is configured for main mode. The admin sees 'no proposal chosen' in the IKE debug. What is the MOST likely cause?
medium
A.The pre-shared key is incorrect
✓ B.The IKE mode (main vs aggressive) does not match between peers
C.The local firewall is blocking UDP port 500
D.The Phase 2 encryption algorithm is not supported
Why B: Option D is correct. IKE mode (main vs aggressive) must match between peers. If one side uses main mode and the other aggressive mode, Phase 1 will fail with 'no proposal chosen' because the IKE exchange format differs.
Last reviewed: Jun 21, 2026
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.
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.
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.