- A
The Phase 2 proposal uses AES256 but the remote client only supports AES128
Why wrong: If Phase 2 is up, the proposals matched. This would cause Phase 2 to fail, not traffic issues after tunnel is up.
- B
The FortiGate's routing table does not have a route to the remote user's subnet
Why wrong: For dial-up VPN, the remote user's IP is typically dynamic, and the FortiGate relies on the tunnel interface route. However, the absence of sessions indicates a policy drop.
- C
There is no firewall policy permitting traffic from the dial-up interface to the destination network
A common issue after VPN establishment is missing firewall policies. Even with the tunnel up, traffic is dropped unless a policy allows it.
- D
The remote user's FortiClient is blocking split tunneling
Why wrong: Split tunneling affects routing, but if the tunnel is up and no sessions are seen, it suggests a policy issue on the FortiGate.
NSE4 Authentication and VPN Practice Question
This NSE4 practice question tests your understanding of authentication and vpn. 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 dial-up IPsec VPN where remote users can connect but traffic does not pass. The Phase 1 and Phase 2 status show 'up'. The administrator runs 'diagnose vpn tunnel list' and sees the tunnel is up. However, 'diagnose sys session list' shows no sessions for the remote user's IP. 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 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
There is no firewall policy permitting traffic from the dial-up interface to the destination network
Option D is correct. Even though the VPN tunnel is up, if there is no firewall policy allowing traffic from the dial-up interface to the internal network, the traffic will be dropped silently.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 Phase 2 proposal uses AES256 but the remote client only supports AES128
Why it's wrong here
If Phase 2 is up, the proposals matched. This would cause Phase 2 to fail, not traffic issues after tunnel is up.
- ✗
The FortiGate's routing table does not have a route to the remote user's subnet
Why it's wrong here
For dial-up VPN, the remote user's IP is typically dynamic, and the FortiGate relies on the tunnel interface route. However, the absence of sessions indicates a policy drop.
- ✓
There is no firewall policy permitting traffic from the dial-up interface to the destination network
- ✗
The remote user's FortiClient is blocking split tunneling
Why it's wrong here
Split tunneling affects routing, but if the tunnel is up and no sessions are seen, it suggests a policy issue on the FortiGate.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
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Authentication and VPN — study guide chapter
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FAQ
Questions learners often ask
What does this NSE4 question test?
Authentication and VPN — This question tests Authentication and VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: There is no firewall policy permitting traffic from the dial-up interface to the destination network — Option D is correct. Even though the VPN tunnel is up, if there is no firewall policy allowing traffic from the dial-up interface to the internal network, the traffic will be dropped silently.
What should I do if I get this NSE4 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
About these practice questions
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Last reviewed: Jun 21, 2026
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.
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