NSE7 Advanced VPN and Zero Trust Practice Question
A FortiGate administrator is configuring an IPsec VPN with IKEv2. The administrator wants to ensure that the VPN tunnel uses perfect forward secrecy (PFS) for phase 2. Which parameter must be configured in the phase 2 proposal?
⚠ Common exam trap
The trap here is thinking that a shorter phase 2 keylife or stronger encryption provides perfect forward secrecy, when only a DH exchange does.
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
✓
Enable 'set pfs enable' and select a Diffie-Hellman group.
To enable perfect forward secrecy in an IPsec VPN phase 2, the administrator must enable PFS and specify a Diffie-Hellman group in the phase 2 proposal. This forces a new key exchange for each phase 2 rekey, ensuring that session keys are not derived from the phase 1 key. Both peers must have matching PFS settings for the tunnel to establish.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure 'set auto-negotiate enable' in the phase 2 proposal.
Why it's wrong here
Auto-negotiate allows the FortiGate to initiate phase 2 negotiations, but it does not enable PFS. PFS is a cryptographic property that requires explicit DH group configuration. This option is unrelated to the requirement and would not provide the desired security.
- ✓
Enable 'set pfs enable' and select a Diffie-Hellman group.
Why this is correct
In the phase 2 proposal, enabling PFS and selecting a DH group ensures that a new key exchange occurs for each phase 2 rekey, providing perfect forward secrecy. This setting must match on both peers. Without it, the tunnel uses the phase 1 keys, which reduces security.
- ✗
Use a stronger encryption algorithm such as AES-256 in phase 2.
Why it's wrong here
Using AES-256 strengthens encryption but does not provide PFS. PFS ensures that compromise of one key does not compromise past or future keys. Encryption algorithm choice is independent of PFS. This option addresses confidentiality but not forward secrecy.
- ✗
Set the phase 2 keylife to a value lower than the phase 1 keylife.
Why it's wrong here
Setting a lower phase 2 keylife does not provide PFS. It only controls how often phase 2 rekeys occur. PFS requires a new DH exchange, which is independent of key lifetimes. This option is a common misconception but does not achieve the security goal.
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
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