NSE7 Advanced VPN and Zero Trust Practice Question
A network administrator is troubleshooting an IPsec VPN between two FortiGates. The phase1 is up, but phase2 keeps failing to establish. The administrator runs 'diagnose vpn ike log' and sees: 'no proposal chosen'. Both sides have the same phase2 configuration: AES256-SHA256, DH group 14, 3600 seconds lifetime. 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
✓
The phase2 local and remote subnets do not match on both sides
Even if the encryption/authentication proposals match, a common issue is a mismatch in the local and remote subnets (selectors). The phase2 negotiation requires matching traffic selectors. If one side has 192.168.1.0/24 and the other has 10.0.0.0/8, the proposals will be rejected. Option C is correct.
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 NAT traversal setting is inconsistent
Why it's wrong here
NAT-T affects phase1 and encapsulation, not phase2 proposal matching.
- ✗
The IKE version is different on each side
Why it's wrong here
If phase1 is established, IKE version matches.
- ✓
The phase2 local and remote subnets do not match on both sides
Why this is correct
The 'no proposal chosen' error in phase2 usually indicates a mismatch in the traffic selectors (subnets). Both sides must have mirroring subnet definitions.
- ✗
The pre-shared key is incorrect
Why it's wrong here
Phase1 is up, so the PSK is correct.
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.