Quick Answer
The answer is a hash algorithm mismatch between the two FortiGate peers. This failure occurs because IKEv2 phase 1 negotiation requires an exact match on every parameter in the proposal, including the hash algorithm; the headquarters uses SHA256 while the remote office uses SHA1, so the IKE SA cannot be established, leaving the tunnel stuck in a perpetual 'negotiating' state. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this scenario tests your understanding of IPsec proposal matching and the common pitfall of assuming only encryption and DH group need to align—especially after a FortiOS upgrade that may change default settings. A frequent trap is overlooking the hash algorithm when verifying configurations, as the pre-shared key and IKE version may appear identical. Remember the memory tip: “Hash must match, or the tunnel stays in the catch.”
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 company has a FortiGate at headquarters running FortiOS 7.2 and a remote office with a FortiGate 60F running FortiOS 7.0. They have an IPsec VPN tunnel between them for site-to-site connectivity. Recently, the remote office upgraded their FortiGate from 6.4 to 7.0. After the upgrade, the VPN tunnel is down. The Phase 1 status shows 'negotiating' but never completes. The administrator has verified that the pre-shared key, IKE version (IKEv2), and authentication method are the same on both sides. The Phase 1 proposal on the headquarters is: encryption: AES256, SHA256, DH group 14, lifetime 86400. The remote office uses: encryption: AES256, SHA1, DH group 14, lifetime 86400. What is the most likely cause of the failure?
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.
Clue: "never"
Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
The DH group is different; headquarters uses group 14, remote uses group 5.
Why wrong: Both use group 14.
The Phase 1 hash algorithm differs; headquarters uses SHA256, remote uses SHA1.
Hash algorithm must match.
The IKE version is mismatched; headquarters uses IKEv2 and remote uses IKEv1.
Why wrong: Both are IKEv2.
The pre-shared key is incorrect after the upgrade.
Why wrong: The administrator verified it.
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 Phase 1 hash algorithm differs; headquarters uses SHA256, remote uses SHA1.
The Phase 1 proposal mismatch on the hash algorithm (SHA256 vs. SHA1) prevents the IKEv2 peers from agreeing on a common transform set. Even though all other parameters match, the hash algorithm must be identical on both sides for the IKE SA to be established. The 'negotiating' state that never completes is a classic symptom of a proposal mismatch.
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 DH group is different; headquarters uses group 14, remote uses group 5.
Why it's wrong here
Both use group 14.
The Phase 1 hash algorithm differs; headquarters uses SHA256, remote uses SHA1.
Why this is correct
Hash algorithm must match.
Clue confirmation
The clue words "most likely", "never" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The IKE version is mismatched; headquarters uses IKEv2 and remote uses IKEv1.
Why it's wrong here
Both are IKEv2.
The pre-shared key is incorrect after the upgrade.
Why it's wrong here
The administrator verified it.
Common exam traps
The trap here is that candidates assume all Phase 1 parameters are correct because the pre-shared key, IKE version, and authentication method match, overlooking the critical requirement that the hash algorithm must also be identical for the IKE SA to be established.
Detailed technical explanation
In IKEv2, the initiator sends a proposal containing one or more transform sets; the responder selects a matching transform set. The hash algorithm (integrity algorithm) is part of the transform set and must match exactly. FortiGate logs (e.g., 'IKE negotiation failed due to proposal mismatch') can be viewed via 'diagnose debug application ike -1' to confirm the specific mismatch. In real-world scenarios, administrators often overlook hash algorithm consistency when upgrading firmware, as default proposals may change between versions.
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 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
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Authentication and VPN — This question tests Authentication and VPN — Read the scenario before looking for a memorised answer..
The correct answer is: The Phase 1 hash algorithm differs; headquarters uses SHA256, remote uses SHA1. — The Phase 1 proposal mismatch on the hash algorithm (SHA256 vs. SHA1) prevents the IKEv2 peers from agreeing on a common transform set. Even though all other parameters match, the hash algorithm must be identical on both sides for the IKE SA to be established. The 'negotiating' state that never completes is a classic symptom of a proposal mismatch.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely", "never". 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 11, 2026
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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