The answer is a missing static route pointing to the VPN tunnel interface. When a VPN tunnel is up with matching Phase 2 selectors but shows zero bytes in both directions, the issue is almost always that the FortiGate does not know to send traffic into the tunnel. The Phase 2 security association is correctly established, but without a route directing the remote subnet’s traffic to the tunnel interface, the firewall drops or forwards the packets out the wrong interface. On the Fortinet NSE 7 Advanced Security NSE7 exam, this scenario tests your understanding that a tunnel being up is a separate condition from traffic actually flowing through it—a common trap is to focus on Phase 2 parameters when the real culprit is routing. Remember the mnemonic: “Tunnel up, no bytes? Check your routes for the remote sites.”
NSE7 Troubleshooting and Diagnostics Practice Question
This NSE7 practice question tests your understanding of troubleshooting and diagnostics. 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 up but no traffic passes through. The Phase 2 selectors match. The administrator runs 'diagnose vpn tunnel list' and sees that the tunnel has '0 bytes' in both directions. 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
✓
The static route for the remote subnet does not point to the VPN tunnel interface
Option B is correct. A correct Phase2 SA but zero traffic indicates that routing is not directing traffic into the tunnel. The routes for the remote subnet must point to the 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 firewall policy is not configured to allow traffic through the tunnel
Why it's wrong here
If the policy were missing, the tunnel would still show traffic attempts (blocked). Zero bytes means no traffic reaches the tunnel.
✓
The static route for the remote subnet does not point to the VPN tunnel interface
Why this is correct
Without proper routing, traffic is not sent to the tunnel.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Rekey interval would not cause zero bytes; it would cause periodic renegotiation.
✗
The NAT traversal is not enabled
Why it's wrong here
NAT traversal affects tunnel establishment, not traffic flow after tunnel is up.
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.
Trap categories for this question
Command / output trap
If the policy were missing, the tunnel would still show traffic attempts (blocked). Zero bytes means no traffic reaches the tunnel.
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 NSE7 subnetting questions on CIDR, address ranges, and subnet selection.
Troubleshooting and Diagnostics — This question tests Troubleshooting and Diagnostics — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The static route for the remote subnet does not point to the VPN tunnel interface — Option B is correct. A correct Phase2 SA but zero traffic indicates that routing is not directing traffic into the tunnel. The routes for the remote subnet must point to the tunnel interface.
What should I do if I get this NSE7 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 NSE7 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.
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