The answer is a mismatched encryption or authentication algorithm between the two VPN peers. This is the most likely cause because the IPsec proposal mismatch error occurs when the security parameters proposed by one peer do not match the acceptable parameters on the other peer during IKE or IPsec SA negotiation; the tunnel drops and re-establishes repeatedly as each failed negotiation triggers a retry with the same mismatched proposal. On the Fortinet NSE 7 Advanced Security NSE7 exam, this scenario tests your understanding of IPsec phase 1 and phase 2 parameter alignment, often appearing as a troubleshooting question where logs show repeated SA failures. A common trap is to blame routing or firewall policies first, but the error message itself points directly to algorithm mismatch. Memory tip: think of it as a handshake where both sides must agree on the same cipher and hash—if one says AES256-SHA256 and the other says AES128-SHA1, the handshake fails every time.
NSE7 Advanced VPN and Zero Trust Practice Question
This NSE7 practice question tests your understanding of advanced vpn and zero trust. 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 company's FortiGate is configured with multiple IPsec VPN tunnels to branch offices. One tunnel keeps dropping and re-establishing every few minutes. The logs show 'IPsec SA negotiation failed' with error 'proposal mismatch'. What is the most likely cause?
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
✓
Mismatched encryption or authentication algorithms between the two VPN peers
The error 'proposal mismatch' directly indicates that the two IPsec peers cannot agree on the security parameters for the IKE or IPsec SA. This occurs when the encryption algorithm (e.g., AES256 vs. AES128), authentication algorithm (e.g., SHA256 vs. SHA1), Diffie-Hellman group, or lifetime values do not match between the FortiGate and the remote peer. The tunnel drops and re-establishes because the negotiation fails, and the FortiGate retries with the same mismatched proposal, leading to repeated failures.
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.
✗
Dead Peer Detection (DPD) configured too aggressively
Why it's wrong here
DPD only monitors peer liveness; it doesn't cause proposal mismatch.
✓
Mismatched encryption or authentication algorithms between the two VPN peers
Why this is correct
Proposal mismatch directly indicates algorithms or parameters don't match.
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.
✗
NAT-Traversal (NAT-T) not enabled
Why it's wrong here
NAT-T affects encapsulation, not proposal matching.
✗
Pre-shared key mismatch
Why it's wrong here
Incorrect PSK results in authentication failure, not proposal mismatch.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse 'proposal mismatch' with authentication failures (pre-shared key) or connectivity issues (NAT-T/DPD), but the specific log message 'proposal mismatch' is a direct indicator of cryptographic parameter disagreement, not a key or transport layer problem.
Detailed technical explanation
How to think about this question
The IPsec proposal mismatch is checked during IKE Phase 1 (for IKE SA) and Phase 2 (for IPsec SA). The FortiGate sends a list of proposals (e.g., encryption, hash, DH group, lifetime) in the SA payload, and the responder must select a matching one from its configured policy. If no match exists, the responder sends a 'NO_PROPOSAL_CHOSEN' notification, which is logged as 'proposal mismatch'. In real-world scenarios, this often happens after firmware upgrades where default algorithms change (e.g., from 3DES to AES) or when one peer uses a custom proposal set that the other does not support.
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.
TExam Day Tips
→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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Advanced VPN and Zero Trust — This question tests Advanced VPN and Zero Trust — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mismatched encryption or authentication algorithms between the two VPN peers — The error 'proposal mismatch' directly indicates that the two IPsec peers cannot agree on the security parameters for the IKE or IPsec SA. This occurs when the encryption algorithm (e.g., AES256 vs. AES128), authentication algorithm (e.g., SHA256 vs. SHA1), Diffie-Hellman group, or lifetime values do not match between the FortiGate and the remote peer. The tunnel drops and re-establishes because the negotiation fails, and the FortiGate retries with the same mismatched proposal, leading to repeated failures.
What should I do if I get this NSE7 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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