NSE4 Authentication and VPN Practice Question
You run the following command on a FortiGate: diagnose vpn ike gateway list. The output shows a gateway with state=DOWN. What is the most likely cause?
⚠ Common exam trap
Candidates often confuse Phase 1 (IKE) failures with Phase 2 (IPsec) issues, but state=DOWN indicates a Phase 1 problem. However, certificate trust errors are also Phase 1 failures; they are not excluded by state=DOWN, but the most likely cause among the options is remote peer unreachable/blocking IKE traffic.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The remote peer is not reachable or is blocking IKE traffic
The `state=DOWN` in the `diagnose vpn ike gateway list` output indicates that the IKE Phase 1 (main mode or aggressive mode) negotiation has failed or never completed. The most common cause is that the remote peer is unreachable (e.g., due to network issues, firewall rules blocking UDP ports 500/4500, or incorrect peer IP configuration) or that the remote peer is actively blocking IKE traffic, preventing the initial exchange of IKE SA proposals.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The remote peer is not reachable or is blocking IKE traffic
Why this is correct
The `diagnose vpn ike` output showing an IKE SA state of DOWN typically indicates that no IKE negotiation response has been received. This commonly means the remote peer is unreachable at the network layer (no route, filter, or the peer is down) or that IKE traffic on UDP ports 500/4500 is being blocked by an intermediate firewall. Before investigating authentication or configuration mismatches, you must verify basic IP connectivity and bidirectional UDP reachability to the peer's public IP address.
- ✗
The pre-shared key is correct but expired
Why it's wrong here
Pre-shared keys (PSKs) are static shared secrets configured on both peers; they do not possess a validity period or expiration date. A mismatch in the PSK would generate an IKE authentication failure during Phase 1, resulting in a DOWN state, but the term 'expired' is semantically incorrect for a PSK. Certificate-based authentication has expiration, not pre-shared keys, so this option can be dismissed as a misunderstanding of IKE credential types.
- ✗
The IPsec Phase 2 parameters are mismatched
Why it's wrong here
IPsec Phase 2 parameters (such as proxy IDs, ESP encryption/integrity algorithms, and traffic selectors) are negotiated only after Phase 1 has successfully completed and an IKE SA is established. When `diagnose vpn ike` shows a down Phase 1 state, Phase 2 has never been reached, so a Phase 2 mismatch cannot be the cause of the initial DOWN. A Phase 2 issue would typically manifest as a negative security association lookup or no SAs while the IKE SA remains up.
- ✗
The local certificate is not trusted by the remote peer
Why it's wrong here
If the local certificate is not trusted by the remote peer, IKE authentication will fail during Phase 1, producing a DOWN state; however, this requires that the peers can first exchange IKE packets successfully. Network reachability and UDP port 4500/500 connectivity are prerequisites for any certificate validation to occur, making a certificate trust issue a secondary consideration after verifying basic connectivity. Furthermore, the prompt asks for the most likely cause, and unreachability or firewall blocking is a far more common and basic failure than certificate chain validation problems.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.