NSE4 Authentication and VPN Practice Question
A network administrator is configuring a site-to-site IPsec VPN between two FortiGates. Phase 1 and Phase 2 are both up, but traffic is not passing through the tunnel. The administrator runs 'diagnose debug flow' and sees that packets are being dropped with the message 'iprope_in_check() check failed, drop'. Which configuration change is most likely to resolve this issue?
⚠ Common exam trap
The trap here is focusing on Phase 2 or routing when the error clearly indicates a firewall policy drop, so the missing policy is the actual cause.
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
✓
Add a firewall policy that allows traffic from the local subnet to the remote subnet through the IPsec tunnel.
The error 'iprope_in_check() check failed, drop' is generated by the FortiGate when a packet is denied by a firewall policy. For IPsec VPN traffic to pass, a firewall policy must allow traffic from the local subnet to the remote subnet, with the correct source and destination interfaces. Even if Phase 1 and Phase 2 are up, without this policy, the FortiGate will drop the packets. The administrator should verify that the policy exists and is correctly ordered.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Add a firewall policy that allows traffic from the local subnet to the remote subnet through the IPsec tunnel.
Why this is correct
The 'iprope_in_check() check failed' error indicates that the FortiGate is dropping packets due to a missing or misconfigured firewall policy. For traffic to pass through the IPsec tunnel, a firewall policy must explicitly allow traffic from the local subnet to the remote subnet, with the correct incoming and outgoing interfaces. Without this policy, the FortiGate drops the packets.
- ✗
Modify the Phase 2 selector to match the remote subnet.
Why it's wrong here
If Phase 2 selectors did not match, Phase 2 would not come up. The error 'iprope_in_check() check failed' indicates a firewall policy issue, not a Phase 2 mismatch. The administrator should verify the firewall policies that permit traffic between the local and remote subnets.
- ✗
Configure a static route for the remote subnet pointing to the IPsec tunnel interface.
Why it's wrong here
A static route is necessary for traffic to be directed into the tunnel, but the error 'iprope_in_check() check failed' is specifically a firewall policy check failure, not a routing issue. If the route were missing, the packet would not even reach the policy check. Therefore, the missing firewall policy is the more likely cause.
- ✗
Enable 'auto-negotiate' in Phase 2 settings to allow dynamic routing.
Why it's wrong here
'Auto-negotiate' is used for dial-up VPNs to allow the remote gateway to be dynamic. In a site-to-site VPN with static IPs, this setting is not required. The error is not related to Phase 2 negotiation; it is a firewall policy drop, so enabling auto-negotiate would not resolve the issue.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Fortinet exam blueprint
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.