- A
The reverse path forwarding (RPF) check is dropping the incoming SYN.
Why wrong: If RPF dropped the SYN, it would not appear in the session table as SYN_RECV.
- B
The firewall policy is not configured with the correct source interface.
Why wrong: The policy allows traffic; otherwise the SYN would be dropped.
- C
The BGP route advertisement is being filtered by a route map on the partner side.
Why wrong: If BGP is up and advertising, filtering would prevent the route from being learned, but the partner is sending traffic.
- D
The Phase 2 selector for the VPN does not include the new subnet, so the FortiGate does not encrypt the reply traffic.
The SYN is received but the return traffic cannot be encrypted because the Phase 2 selector does not match the new subnet, causing asymmetric routing.
Quick Answer
The answer is a Phase 2 selector mismatch. Even though BGP successfully advertises the new subnet 192.168.50.0/24 and the routing table shows the route, the FortiGate will not encrypt reply traffic unless that subnet is explicitly included in the Phase 2 selector definition for the VPN tunnel. This is why the packet capture shows the SYN arriving but no SYN-ACK being sent—the firewall sees the incoming request but cannot find a matching Phase 2 proposal to encrypt the response, leaving sessions stuck in SYN_RECV state. On the Fortinet NSE 7 Advanced Security NSE7 exam, this scenario tests your understanding that route-based VPNs still require Phase 2 selectors to define encryption boundaries; BGP handles routing, not encryption policy. A common trap is assuming a BGP-advertised route automatically enables encryption, but the Phase 2 selector acts as an independent traffic filter. Remember: BGP tells the router *where* to send traffic, but Phase 2 tells the firewall *whether* to encrypt it.
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 company runs a FortiGate 600E in NAT/Route mode. They have a site-to-site VPN to a partner using route-based VPN with BGP. Recently, they added a new subnet 192.168.50.0/24 behind the FortiGate. The BGP session is up, and the route is being advertised to the partner. However, traffic from the partner to the new subnet fails. The FortiGate's routing table shows the route to 192.168.50.0/24 is present via the VPN interface. Firewall policies allow the traffic. A packet capture on the FortiGate's internal interface shows the partner's traffic arriving but no SYN-ACK being sent back. The FortiGate's session table shows sessions in 'SYN_RECV' state for the new subnet. 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
The Phase 2 selector for the VPN does not include the new subnet, so the FortiGate does not encrypt the reply traffic.
The correct answer is D. In a route-based VPN, the Phase 2 selectors define which traffic is allowed to be encrypted and decrypted. Even though BGP advertises the new subnet 192.168.50.0/24, if the Phase 2 selector on either side does not include this subnet, the FortiGate will not encrypt the reply traffic. The packet capture shows the SYN arriving, but no SYN-ACK is sent because the FortiGate cannot find a matching Phase 2 proposal to encrypt the response, causing the session to remain in SYN_RECV state.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 reverse path forwarding (RPF) check is dropping the incoming SYN.
Why it's wrong here
If RPF dropped the SYN, it would not appear in the session table as SYN_RECV.
- ✗
The firewall policy is not configured with the correct source interface.
Why it's wrong here
The policy allows traffic; otherwise the SYN would be dropped.
- ✗
The BGP route advertisement is being filtered by a route map on the partner side.
Why it's wrong here
If BGP is up and advertising, filtering would prevent the route from being learned, but the partner is sending traffic.
- ✓
The Phase 2 selector for the VPN does not include the new subnet, so the FortiGate does not encrypt the reply traffic.
Why this is correct
The SYN is received but the return traffic cannot be encrypted because the Phase 2 selector does not match the new subnet, causing asymmetric routing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume BGP route advertisement alone ensures traffic flow, overlooking that IPsec Phase 2 selectors must explicitly match the new subnet for encryption to occur.
Detailed technical explanation
How to think about this question
In FortiGate route-based VPNs, Phase 2 selectors act as a traffic filter for IPsec encryption. When a new subnet is added, the Phase 2 selector must be updated or a new Phase 2 entry created to include the new subnet; otherwise, the FortiGate will not encrypt packets destined to that subnet, even if routing and firewall policies are correct. The SYN_RECV state indicates the FortiGate received the SYN and created a session, but the TCP stack cannot send the SYN-ACK because the outbound interface (VPN tunnel) has no valid IPsec SA for the destination subnet, causing the packet to be dropped silently.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
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FAQ
Questions learners often ask
What does this NSE7 question test?
Troubleshooting and Diagnostics — This question tests Troubleshooting and Diagnostics — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Phase 2 selector for the VPN does not include the new subnet, so the FortiGate does not encrypt the reply traffic. — The correct answer is D. In a route-based VPN, the Phase 2 selectors define which traffic is allowed to be encrypted and decrypted. Even though BGP advertises the new subnet 192.168.50.0/24, if the Phase 2 selector on either side does not include this subnet, the FortiGate will not encrypt the reply traffic. The packet capture shows the SYN arriving, but no SYN-ACK is sent because the FortiGate cannot find a matching Phase 2 proposal to encrypt the response, causing the session to remain in SYN_RECV state.
What should I do if I get this NSE7 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
This NSE7 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 NSE7 exam.
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