Quick Answer
The answer is a Phase 2 selector mismatch between the local and remote subnets. When Phase 1 parameters are identical and the VPN status shows 'down', the most likely cause is that the Phase 2 selectors—the specific source and destination subnet definitions—do not match exactly on both FortiGates. Phase 2 uses these selectors to negotiate IPsec security associations; if they are misaligned, the Quick Mode or Child SA exchange fails, leaving the tunnel down even if the IKE Phase 1 SA is up. On the Fortinet NSE 4 exam, this scenario tests your understanding of IPsec VPN troubleshooting, often appearing as a trap where you might overlook subnet mismatches after confirming Phase 1. A common memory tip is to think of Phase 2 selectors as a "mirror"—they must be exact opposites on each side, so always verify that the local subnet on one FortiGate matches the remote subnet on the other, and vice versa.
This NSE4 practice question tests your understanding of authentication and vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configuring a site-to-site IPsec VPN between two FortiGates. After applying the configuration, the VPN status shows 'down'. Phase 1 parameters are identical on both sides. 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.
The Phase 2 selectors (local and remote subnets) are mismatched.
Phase 2 requires matching proxy IDs.
The pre-shared keys do not match.
Why wrong: Phase 1 would fail, not Phase 2.
The firewall policies are not configured.
Why wrong: Policies are needed for traffic, not for bringing up the tunnel.
NAT traversal is disabled but both FortiGates are behind NAT.
Why wrong: NAT-T mismatch would cause Phase 1 issues.
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 2 selectors (local and remote subnets) are mismatched.
When Phase 1 parameters are identical and the VPN is down, the most common cause is a mismatch in Phase 2 selectors (local and remote subnets). Phase 2 uses these selectors to negotiate the IPsec security associations (SAs); if they do not match exactly on both sides, the IKEv1/v2 Quick Mode or Child SA exchange will fail, leaving the tunnel in a 'down' state even though Phase 1 (IKE SA) may be up.
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 Phase 2 selectors (local and remote subnets) are mismatched.
Why this is correct
Phase 2 requires matching proxy IDs.
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.
The pre-shared keys do not match.
Why it's wrong here
Phase 1 would fail, not Phase 2.
The firewall policies are not configured.
Why it's wrong here
Policies are needed for traffic, not for bringing up the tunnel.
NAT traversal is disabled but both FortiGates are behind NAT.
Why it's wrong here
NAT-T mismatch would cause Phase 1 issues.
Common exam traps
The trap here is that candidates often assume a Phase 1 mismatch (like pre-shared keys) is the cause when the VPN is down, but the question explicitly states Phase 1 parameters are identical, forcing the focus to Phase 2 selector mismatches, which is a classic NSE4 exam trick.
Detailed technical explanation
In IPsec, Phase 2 (Quick Mode for IKEv1 or Child SA for IKEv2) negotiates the encryption and authentication parameters for data traffic, and the selectors define which subnets are allowed to communicate. If the local subnet on one side does not match the remote subnet on the other, the IPsec SA negotiation will reject the proposal, causing the tunnel to remain down. A subtle behavior is that FortiGate logs may show 'no matching proposal' in the Phase 2 debug, and the tunnel may appear 'up' in Phase 1 but 'down' in Phase 2 status.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Authentication and VPN — study guide chapter
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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 2 selectors (local and remote subnets) are mismatched. — When Phase 1 parameters are identical and the VPN is down, the most common cause is a mismatch in Phase 2 selectors (local and remote subnets). Phase 2 uses these selectors to negotiate the IPsec security associations (SAs); if they do not match exactly on both sides, the IKEv1/v2 Quick Mode or Child SA exchange will fail, leaving the tunnel in a 'down' state even though Phase 1 (IKE SA) may be up.
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". 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.
About these practice questions
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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A FortiGate administrator has configured a route-based IPsec VPN. After Phase 2 is up, traffic is not passing. The administrator verifies that the firewall policy allows traffic and the routes are correct. What should the administrator check next?
mediumWhy A: In route-based VPN, the VPN interface must have the correct remote IP address or the route must be via the VPN interface. A common issue is that the remote subnet is not correctly learned or the static route points to the correct interface.
Last reviewed: Jun 11, 2026
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