The answer is a missing firewall policy to permit traffic between the local and remote subnets via the virtual tunnel interface. In a route-based IPsec VPN, Phase 1 and Phase 2 establish the encrypted tunnel, and the static route directs traffic into the tunnel interface, but the FortiGate still enforces security policies at the interface level. Without an explicit firewall policy allowing traffic from the source interface (e.g., internal) to the tunnel interface (e.g., 'to_remote'), the FortiGate drops the packets, even though the VPN is technically up. On the Fortinet NSE 4 Network Security Professional exam, this is a classic trap: candidates often assume a route-based VPN only needs routes and IPsec settings, forgetting that firewall policies govern all inter-interface traffic. The exam tests your understanding that route-based VPNs separate routing from security, unlike policy-based VPNs where the policy itself defines the traffic selector. Memory tip: "Tunnel up, route set, but no policy? Packets get wrecked."
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.
An administrator configures a route-based IPsec VPN between two FortiGates. The Phase 1 and Phase 2 are up. The administrator adds a static route on each FortiGate pointing to the remote subnet via the virtual tunnel interface (e.g., 'to_remote'). Traffic between the subnets fails. What is the MOST likely missing configuration?
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
✓
A firewall policy is required to permit traffic between the interfaces
In a route-based VPN, the tunnel interface is part of a zone or has its own security policy. Without a firewall policy allowing traffic from the local subnet to the remote subnet via the tunnel interface, traffic will be dropped.
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.
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.
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: A firewall policy is required to permit traffic between the interfaces — In a route-based VPN, the tunnel interface is part of a zone or has its own security policy. Without a firewall policy allowing traffic from the local subnet to the remote subnet via the tunnel interface, traffic will be dropped.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 is configuring a route-based IPsec VPN between two FortiGate devices. After setting up the tunnel and firewall policies, traffic does not flow. The administrator runs 'diagnose vpn tunnel list' and sees the tunnel is up. 'get router info routing-table all' shows routes on both sides. However, pings from the local network to the remote network fail. What is the MOST likely cause?
hard
A.The pre-shared key is incorrect
✓ B.The firewall policy allowing traffic to the remote subnet has the source and destination interfaces reversed
C.The remote FortiGate's static route points to the wrong local subnet
D.The Phase 2 proposal uses different encryption algorithms on each side
Why B: Option A is correct. For route-based VPN, traffic must be allowed by the policy that has the VPN interface as the destination interface. If the policy's source and destination are reversed (e.g., source internal, destination internal instead of source internal, destination VPN), traffic will be dropped.
Last reviewed: Jun 21, 2026
Question Discussion
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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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