The answer is a missing static route on the FortiGate for the remote subnet pointing to the tunnel interface. Even when an IPsec VPN tunnel is established and a firewall policy is in place, traffic cannot flow without a proper route telling the FortiGate which interface to use to reach the remote network. Routing misconfiguration is the most common cause of the "IPsec VPN tunnel established but no traffic" issue, as the tunnel interface itself is not automatically added to the routing table for remote subnets. On the Fortinet NSE 4 exam, this scenario tests your understanding of the three pillars of VPN traffic flow: phase 1, phase 2, and routing—with routing being the frequently overlooked trap. A common memory tip is "Tunnel up, policy in place, but no route? No ride." Always verify that a static route exists for the remote subnet with the tunnel interface as the destination device.
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 FortiGate administrator notices that the IPsec VPN tunnel is established but traffic is not passing. The firewall policy allowing traffic from the remote subnet to the local subnet is in place. 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 static route on the FortiGate for the remote subnet pointing to the tunnel interface
If the tunnel is up but no traffic passes, it could be due to routing misconfiguration, such as missing static routes for the remote subnet pointing to the VPN tunnel interface.
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 VPN tunnel is a policy-based VPN and the policy is incorrectly configured
Why it's wrong here
Policy-based VPNs require policy match; but if the tunnel is up and policy exists, routing is more likely.
✗
The Phase 2 proposal includes PFS, but the remote side does not
Why it's wrong here
PFS mismatch would prevent Phase 2 from coming up.
✗
The local firewall is blocking ICMP
Why it's wrong here
ICMP blocking would affect ping, but not necessarily all traffic.
✓
There is no static route on the FortiGate for the remote subnet pointing to the tunnel interface
Why this is correct
Without a route, the FortiGate does not know how to forward traffic to the remote subnet even if the tunnel is up.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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: There is no static route on the FortiGate for the remote subnet pointing to the tunnel interface — If the tunnel is up but no traffic passes, it could be due to routing misconfiguration, such as missing static routes for the remote subnet pointing to the VPN tunnel interface.
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. An administrator is troubleshooting an IPsec VPN that is not passing traffic. The Phase 1 and Phase 2 are both up. Which TWO CLI commands can be used to verify the VPN tunnel status and traffic flow? (Choose two.)
medium
✓ A.diagnose vpn tunnel list
B.execute ping-options source
C.diagnose vpn ike config
D.diagnose netlink interface list
✓ E.diagnose sys session list
Why A: The 'diagnose vpn tunnel list' shows the status of IPsec tunnels. 'diagnose sys session list' with filters shows active sessions that may be using the VPN tunnel.
Variation 2. You are configuring a route-based IPsec VPN with BGP over the tunnel. After Phase 2 is up, the BGP session does not establish. You run 'diagnose debug ipsec' and see no errors. What should you check next?
medium
A.Disable anti-replay on the tunnel
B.Enable NAT traversal
✓ C.Ensure the tunnel interface is added to the BGP neighbor configuration
D.Check the Phase 1 proposal
Why C: In route-based VPN, the tunnel interface must be included in the BGP configuration as a neighbor update source or the BGP update must be allowed in the firewall policy.
Last reviewed: Jun 21, 2026
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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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