NSE7 Advanced VPN and Zero Trust Practice Question
A network administrator has configured an IPsec VPN between two FortiGates using IKEv2 with pre-shared keys. The tunnel establishes successfully, but after a few minutes, traffic stops passing through. The administrator runs 'diagnose vpn ike log' and sees 'DPD timeout' messages. What is the most likely cause of this issue?
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 remote FortiGate is behind a NAT device without proper NAT-T configuration
DPD timeout indicates that the remote peer is not responding to Dead Peer Detection probes. The most common cause is a misconfigured firewall on the path dropping UDP 500 or 4500 packets, or NAT keepalive issues if NAT is involved. Option A is correct because if the remote FortiGate is behind a NAT device without proper NAT-T configuration, the DPD packets may be dropped or not reach the peer, causing the timeout.
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 FortiGate is behind a NAT device without proper NAT-T configuration
Why this is correct
NAT-T is required when one peer is behind NAT. Without it, DPD packets may be dropped, causing the tunnel to be considered dead.
- ✗
The IPsec phase2 proposal is mismatched, causing rekey failures
Why it's wrong here
Phase2 mismatch would cause the tunnel to not establish or fail during rekey, not DPD timeout specifically.
- ✗
The IKE SA lifetime is set too long, causing the tunnel to expire
Why it's wrong here
A long SA lifetime would not cause DPD timeout; it would cause the tunnel to stay up longer.
- ✗
The local FortiGate's DPD interval is set too low, causing false positives
Why it's wrong here
A low DPD interval would cause more frequent probes, but if the remote is reachable, it should respond. This would not cause timeouts unless the remote is unreachable.
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 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.