The answer is to verify that firewall policies permit the VPN traffic, specifically UDP ports 500 and 4500. This is correct because even when Phase 1 parameters match perfectly, the FortiGate must have explicit security policies allowing IKE and ESP/AH traffic to and from the tunnel interfaces; without these policies, the VPN negotiation packets are dropped before they can reach the VPN daemon. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding that VPN establishment is a two-layer process—matching Phase 1 parameters is necessary but not sufficient, and the common trap is to overlook the firewall policy layer after confirming cryptographic settings. A reliable memory tip is “Phase 1 matches, but policies block the matches,” reminding you that IKE traffic (UDP 500/4500) must be explicitly permitted on both the local and remote interfaces for the tunnel to come up.
NSE4 System and Network Administration Practice Question
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.
A FortiGate administrator is troubleshooting a VPN tunnel that is not coming up. The phase 1 parameters match on both sides. Which three configuration items should the administrator verify?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The local and remote interface IP addresses are correctly configured.
Option C is correct because the local and remote interface IP addresses define the tunnel endpoints. If these are misconfigured, the VPN cannot establish a secure connection even if all other parameters match. The FortiGate uses these addresses to route IKE traffic to the correct peer.
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.
✗
NAT traversal is enabled on both sides.
Why it's wrong here
NAT-T is only needed if there is NAT between peers.
✗
The phase 2 proposal matches the remote device.
Why it's wrong here
Phase 2 is only negotiated after phase 1 is up.
✓
The local and remote interface IP addresses are correctly configured.
Read the scenario before looking for a memorised answer.
✓
The pre-shared key is identical on both sides.
Why this is correct
Mismatched PSK causes authentication failure.
Related concept
Read the scenario before looking for a memorised answer.
✓
Firewall policies permit the VPN traffic (UDP 500, 4500).
Why this is correct
If blocked, IKE cannot establish.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus only on phase 1 parameters like encryption and authentication, forgetting that basic connectivity items like interface IPs, pre-shared keys, and firewall rules are equally critical for the tunnel to come up.
Detailed technical explanation
How to think about this question
Phase 1 (IKE SA) uses UDP ports 500 and 4500 for key exchange and authentication. The pre-shared key must be identical on both sides because it is used to generate the authentication hash; a mismatch causes an IKE authentication failure. Firewall policies must permit UDP 500 and 4500 traffic between the tunnel endpoints; if these ports are blocked, IKE packets are dropped and the tunnel never initiates.
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.
System and Network Administration — This question tests System and Network Administration — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The local and remote interface IP addresses are correctly configured. — Option C is correct because the local and remote interface IP addresses define the tunnel endpoints. If these are misconfigured, the VPN cannot establish a secure connection even if all other parameters match. The FortiGate uses these addresses to route IKE traffic to the correct peer.
What should I do if I get this NSE4 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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