The answer is a missing firewall policy allowing traffic through the VPN tunnel. Even when a site-to-site IPsec VPN establishes successfully using IKEv1 main mode, the tunnel itself only creates a secure communication path; it does not automatically permit data to flow. FortiGate firewalls operate on a policy-based model, meaning that traffic entering the tunnel interface must be explicitly allowed by a firewall policy that matches the source and destination zones, or the traffic is silently dropped. On the Fortinet NSE 4 Network Security Professional NSE4 exam, this is a classic trap—candidates often assume a green tunnel icon means traffic is passing, but the exam tests your understanding that IPsec phase 1 and phase 2 only handle encryption and authentication, not traffic forwarding. The most likely cause of “site-to-site IPsec VPN established but no traffic” is therefore the absence of that critical policy. Memory tip: “Tunnel up, traffic stuck? Check the policy—no policy, no packet.”
NSE4 Authentication and VPN Practice Question
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.
A network admin configures a site-to-site IPsec VPN between two FortiGates using IKEv1 main mode. The tunnel establishes successfully, but no traffic passes. What is the MOST likely cause?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
There is no firewall policy allowing traffic through the VPN tunnel
Main mode IKEv1 uses IP protocol 50 (ESP) and UDP 500/4500. If there is no firewall policy allowing traffic between the source and destination zones, traffic will be dropped even if the tunnel is up.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Aggressive mode should be used instead of main mode
Why it's wrong here
Mode type does not affect traffic flow after tunnel establishment.
✗
The pre-shared key is incorrect
Why it's wrong here
An incorrect PSK would prevent phase1 from completing.
✓
There is no firewall policy allowing traffic through the VPN tunnel
Why this is correct
A firewall policy must explicitly permit traffic from the source to destination zone using the VPN interface.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The phase2 proposal does not match between peers
Why it's wrong here
If phase2 failed, the tunnel would not show as established.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
If phase2 failed, the tunnel would not show as established.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Authentication and VPN — This question tests Authentication and VPN — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: There is no firewall policy allowing traffic through the VPN tunnel — Main mode IKEv1 uses IP protocol 50 (ESP) and UDP 500/4500. If there is no firewall policy allowing traffic between the source and destination zones, traffic will be dropped even if the tunnel is up.
What should I do if I get this NSE4 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related NSE4 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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