NSE7 Advanced VPN and Zero Trust Practice Question
A FortiGate has multiple IPsec VPNs to different branch offices. The administrator notices that one VPN tunnel is flapping (going up and down repeatedly). From the CLI, 'diagnose vpn ike gateway list' shows the gateway state as 'up' but then quickly goes to 'down'. What is the MOST likely 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
✓
Dead Peer Detection (DPD) retry interval is too short
Tunnel flapping with IKEv2 is often due to DPD mismatches or aggressive DPD retry intervals. If DPD is configured with very short intervals, the tunnel may drop due to transient delays.
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's certificate is expired
Why it's wrong here
Certificate expiry would cause failure during authentication, not flapping.
- ✗
The phase2 proposal is mismatched
Why it's wrong here
Mismatched proposals would prevent the tunnel from ever coming up.
- ✓
Dead Peer Detection (DPD) retry interval is too short
Why this is correct
Aggressive DPD can cause false timeouts and tunnel flapping.
- ✗
The pre-shared key is incorrect
Why it's wrong here
Incorrect PSK would cause authentication failure and no up state.
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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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.