NSE7 Advanced VPN and Zero Trust Practice Question
A FortiGate administrator is configuring an IPsec VPN to a remote peer behind a device that performs NAT. The administrator notices that the tunnel establishes but rekeys fail after the Phase 1 lifetime expires. Which setting should the administrator enable on the FortiGate to allow the IKE negotiation to survive NAT and pass through the NAT device reliably?
⚠ Common exam trap
Many candidates confuse NAT traversal with other IPsec options such as PFS or aggressive mode, which do not solve NAT translation issues.
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
✓
NAT traversal (NAT-T) on the IPsec Phase 1 interface.
NAT-T allows IKE and ESP to be encapsulated in UDP so NAT devices can translate and track the flow. Enabling NAT-T on the Phase 1 interface lets the tunnel establish and rekey reliably when a peer sits behind NAT, which directly addresses the rekey failures observed after the Phase 1 lifetime expires.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Dead Peer Detection (DPD) with an aggressive retry interval on Phase 1.
Why it's wrong here
DPD detects whether a peer is still reachable and can trigger tunnel renegotiation, but it does not enable traffic to traverse a NAT device. Aggressive DPD intervals may cause unnecessary renegotiation and do not address the underlying NAT translation problem. DPD is about liveness detection, not NAT traversal.
- ✗
Aggressive mode for IKE Phase 1 instead of main mode.
Why it's wrong here
Aggressive mode reduces the number of IKE exchanges and is sometimes used when the responder has a dynamic IP, but it does not provide NAT traversal. It also exposes the identity in clear text and is generally discouraged. Switching modes would not fix rekey failures caused by NAT translation of ESP traffic.
- ✓
NAT traversal (NAT-T) on the IPsec Phase 1 interface.
Why this is correct
NAT-T encapsulates IKE and ESP in UDP (typically port 4500) so that NAT devices can translate the traffic and maintain state. When a peer is behind NAT, enabling NAT-T on the Phase 1 interface allows the tunnel to establish and rekey reliably, because the NAT device can track the UDP flow. Without NAT-T, ESP packets may be dropped or mishandled, causing rekey failures.
- ✗
Perfect Forward Secrecy (PFS) on the IPsec Phase 2 interface.
Why it's wrong here
PFS forces a new Diffie-Hellman exchange for each Phase 2 rekey, improving forward secrecy but not solving NAT traversal. Enabling PFS does not help IKE or ESP packets pass through a NAT device, and mismatched PFS settings between peers can actually cause Phase 2 rekey failures. It is unrelated to the NAT survivability requirement described.
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 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.