NSE7 Advanced VPN and Zero Trust Practice Question
An administrator needs to configure a site-to-site IPsec VPN with a remote FortiGate that has a dynamic IP address. Which phase1 parameter must be set to support this?
⚠ Common exam trap
Many exam-takers confuse NAT traversal (which handles NAT devices) with the need for a dynamic IP peer, or they assume certificate-based authentication alone solves the issue, but the key requirement is the ability to identify the peer without a known IP address, which aggressive mode enables.
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
✓
Set mode to aggressive and use a pre-shared key
When the remote FortiGate has a dynamic IP address, the local FortiGate cannot initiate the VPN because it does not know the remote peer's IP. Setting the phase1 mode to aggressive and using a pre-shared key allows the remote peer to initiate the connection by sending its identity (ID) in the first exchange, enabling the local FortiGate to identify and authenticate the peer without requiring a static IP address for the remote side.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable Perfect Forward Secrecy (PFS)
Why it's wrong here
PFS is a security feature for key material, not for dynamic peer support.
- ✗
Enable NAT traversal
Why it's wrong here
NAT traversal is for when a NAT device is between peers, not for dynamic IP support.
- ✗
Use certificate-based authentication
Why it's wrong here
Certificate-based authentication does not solve the problem of a peer with a dynamic IP address, because it still requires the remote FortiGate to be identified by a fixed IP or FQDN in the phase1 configuration to establish the IKE session. It is tempting because certificates are commonly used to authenticate peers without pre-shared keys, and in a hub-and-spoke topology where both peers have static IPs, certificate-based authentication would be a valid choice for strong identity verification.
- ✓
Set mode to aggressive and use a pre-shared key
Why this is correct
Aggressive mode allows the responder to initiate without knowing the peer IP; pre-shared key is used for authentication.
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
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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.