The answer is to check the phase1 and phase2 proposal settings on both ends to ensure they match. This is correct because the "no acceptable proposal" error directly indicates a cryptographic mismatch between the FortiGate and the remote peer, meaning neither side can agree on a common set of encryption, authentication, or Diffie-Hellman parameters. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding of IPsec VPN troubleshooting, specifically how FortiOS upgrades can reset or alter default proposal lists, causing incompatibility with third-party devices. A common trap is immediately re-entering pre-shared keys or restarting the tunnel, but the log explicitly points to proposal negotiation failure. The memory tip is "Match the math first"—always verify the proposal algorithms before touching authentication or routing, as the upgrade often changes default values like AES128 to AES256.
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.
After upgrading FortiOS, an IPsec VPN tunnel fails to come up. The diagnose output shows 'negotiation failed: no acceptable proposal'. The remote peer is a third-party device. Which step should you take first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the phase1 and phase2 proposal settings on both ends to ensure they match
The log indicates proposal mismatch. Since the remote peer is third-party, the FortiGate should be configured to match the remote peer's proposals. Checking the phase1 and phase2 settings against the remote peer's requirements is the logical first step.
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.
✗
Change the IKE version to IKEv2
Why it's wrong here
Changing version may not help if the remote peer only supports IKEv1.
✗
Disable dead peer detection on the FortiGate
Why it's wrong here
DPD is not related to proposal negotiation.
✗
Reboot the remote peer
Why it's wrong here
Rebooting does not fix proposal mismatch.
✓
Check the phase1 and phase2 proposal settings on both ends to ensure they match
Why this is correct
Mismatched parameters like encryption, hash, DH group are common causes.
Clue confirmation
The clue word "first" 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.
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: Check the phase1 and phase2 proposal settings on both ends to ensure they match — The log indicates proposal mismatch. Since the remote peer is third-party, the FortiGate should be configured to match the remote peer's proposals. Checking the phase1 and phase2 settings against the remote peer's requirements is the logical first step.
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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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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