NSE4 Authentication and VPN Practice Question
A FortiGate administrator runs 'diagnose vpn tunnel list' and sees the following output for an IPsec tunnel: 'status: up', 'incoming: 0 packets', 'outgoing: 100 packets'. Phase 1 and Phase 2 both show state 'up'. What is the MOST likely cause of zero incoming packets?
⚠ Common exam trap
The trap here is that candidates see 'status: up' and assume the tunnel is fully functional, overlooking that Phase 2 proxy ID mismatches can leave the tunnel up but unable to pass traffic in one direction.
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 proposal includes a mismatched proxy ID
The output shows the VPN tunnel is up with outgoing packets but zero incoming packets. This indicates a Phase 2 mismatch, most commonly due to mismatched proxy IDs (local/remote subnets). When proxy IDs do not match between peers, the tunnel establishes but traffic is not correctly matched, causing the remote gateway to drop or not send traffic to the FortiGate, resulting in zero incoming packets.
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 remote gateway is using aggressive mode
Why it's wrong here
Aggressive mode is a Phase 1 IKE negotiation setting that determines how the preshared key is used and whether identities are exchanged in the clear. Once the Phase 1 tunnel is established, aggressive mode has no influence on packet flow or Phase 2 negotiations. A tunnel in aggressive mode still passes encrypted traffic normally if the Phase 2 selectors are correct, so this would not cause asymmetric or dropped traffic.
- ✗
The FortiGate has a static route pointing to the VPN interface
Why it's wrong here
A static route pointing to the VPN interface is actually required for the FortiGate to route return traffic from the local subnet back to the remote peer. If such a route were missing, replies to incoming packets would be dropped, but having the route does not cause incoming packets to be rejected. This configuration is a prerequisite for bidirectional communication, not a source of packet-flow problems.
- ✗
The VPN is configured in policy-based mode
Why it's wrong here
Policy-based mode relies on firewall policies rather than routes to define the traffic that traverses the IPsec tunnel, but it still permits two-way traffic as long as both directions are permitted in the policy. If a policy is misconfigured, the symptom would be that one direction is blocked, not the mode itself. Therefore, simply being in policy-based mode cannot explain the observed issue; you would look at the policy direction and action.
- ✓
The Phase 2 proposal includes a mismatched proxy ID
Why this is correct
During Phase 2 negotiation, both peers exchange proxy IDs (traffic selectors) that define which source and destination subnets are encrypted. If the local proxy ID does not exactly match the remote peer's expected subnets, the IPsec SA is established with mismatched selectors. The remote gateway may then drop incoming packets that fall outside its configured selectors, or it may not respond at all, causing the tunnel to appear up but traffic to fail in one or both directions.
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.
Go deeper
Related to this question
About these practice questions
This NSE4 question is part of Courseiva's 282-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.