NSE7 Troubleshooting and Diagnostics Practice Question
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?
⚠ Common exam trap
Candidates often assume BGP route advertisement alone ensures traffic flow, overlooking that IPsec Phase 2 selectors must explicitly match the new subnet for encryption to occur.
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.
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.
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.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.