Quick Answer
The answer is the IKE version (v1 or v2) and the pre-shared key (PSK). These two parameters must match on both peers because IKE Phase 1 establishes a secure, authenticated channel; if the IKE version differs, the peers cannot agree on the protocol format for negotiation, and if the PSK does not match, the hash-based authentication fails, triggering the "phase 1 negotiation failed" error in FortiGate logs. On the Fortinet NSE 4 exam, this question tests your ability to isolate the mandatory, non-negotiable Phase 1 parameters from optional settings like encryption algorithms or Diffie-Hellman groups, which can be mismatched if the peers support multiple proposals. A common trap is to assume the PSK is optional or that IKE version is automatically negotiated, but FortiGate requires explicit matching. Memory tip: think "Version and Secret"—the IKE version sets the language, and the PSK is the shared password; both must be identical before any crypto parameters are even considered.
This NSE4 practice question tests your understanding of system and network administration. 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.
An administrator is troubleshooting a loss of connectivity between two sites connected via a VPN tunnel. The FortiGate logs show 'Tunnel: phase 1 negotiation failed'. Which two parameters MUST match on both peers for phase 1 to succeed? (Select two. Not all options are used.)
IPsec proposal (encryption and authentication)
Why wrong: This is for phase 2.
Pre-shared key
The PSK must match on both ends.
IKE version (v1 or v2)
Both must use the same IKE version.
Local ID
Why wrong: Local ID is optional and not required for phase 1 success.
Local and remote IP addresses
Why wrong: These are specified but do not need to be identical; they are peer-specific.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Pre-shared key
The pre-shared key (PSK) is a mandatory authentication method for IKE phase 1. Both peers must use an identical PSK string; a mismatch causes the 'phase 1 negotiation failed' error because the IKE SA cannot be authenticated. FortiGate logs this failure when the calculated hash of the PSK does not match between the two endpoints.
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.
IPsec proposal (encryption and authentication)
Why it's wrong here
This is for phase 2.
Pre-shared key
Why this is correct
The PSK must match on both ends.
Related concept
Read the scenario before looking for a memorised answer.
IKE version (v1 or v2)
Why this is correct
Both must use the same IKE version.
Related concept
Read the scenario before looking for a memorised answer.
Local ID
Why it's wrong here
Local ID is optional and not required for phase 1 success.
Local and remote IP addresses
Why it's wrong here
These are specified but do not need to be identical; they are peer-specific.
Common exam traps
The trap here is that candidates often confuse phase 1 and phase 2 parameters, incorrectly selecting the IPsec proposal (which is a phase 2 parameter) instead of the IKE version, which is a critical phase 1 matching requirement.
Detailed technical explanation
IKE phase 1 establishes a secure, authenticated channel using either main mode or aggressive mode. The mandatory matching parameters include the IKE version (v1 or v2), the pre-shared key (or certificate), and the Diffie-Hellman group, encryption algorithm, hash algorithm, and lifetime values. If any of these differ, the IKE SA negotiation fails; FortiGate logs 'phase 1 negotiation failed' with a specific error code, such as 'mismatched pre-shared key' or 'no proposal chosen'.
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
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FAQ
System and Network Administration — This question tests System and Network Administration — Read the scenario before looking for a memorised answer..
The correct answer is: Pre-shared key — The pre-shared key (PSK) is a mandatory authentication method for IKE phase 1. Both peers must use an identical PSK string; a mismatch causes the 'phase 1 negotiation failed' error because the IKE SA cannot be authenticated. FortiGate logs this failure when the calculated hash of the PSK does not match between the two endpoints.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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