NSE4 Authentication and VPN Practice Question
An administrator is troubleshooting an IPsec VPN between two FortiGates. Phase 1 is up but Phase 2 is down. The admin runs 'diagnose vpn ike log' and sees 'no matching proposal'. To resolve this issue, which TWO settings should be checked on both ends?
⚠ Common exam trap
Watch out — candidates often confuse Phase 1 and Phase 2 parameters, assuming that a Phase 1 mismatch (like encryption algorithm or authentication method) could cause a Phase 2 'no matching proposal' error, when in fact Phase 2 has its own independent set of proposals including PFS and encryption algorithms.
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
✓
Phase 2 PFS (Perfect Forward Secrecy) group
The 'no matching proposal' error in Phase 2 often indicates a mismatch in the Perfect Forward Secrecy (PFS) group. PFS ensures that if a private key is compromised, past session keys are not exposed; both ends must agree on the same Diffie-Hellman group (e.g., group 2, 5, 14) for Phase 2. If one side has PFS enabled with a specific group and the other does not, or uses a different group, the proposal fails.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Phase 2 PFS (Perfect Forward Secrecy) group
Why this is correct
PFS group must match; if one peer has PFS disabled and the other has it enabled, Phase 2 will fail.
- ✗
Phase 1 authentication method
Why it's wrong here
Already negotiated successfully in Phase 1.
- ✗
Phase 2 local and remote subnets
Why it's wrong here
While mismatched subnets can cause issues, the error 'no matching proposal' typically refers to algorithms/PFS, not selectors. However, sometimes selectors are included in the proposal; but the two most common causes are encryption and PFS.
- ✓
Phase 2 encryption algorithm (e.g., AES128, AES256)
Why this is correct
The encryption algorithm must match on both peers for Phase 2 to negotiate.
- ✗
Phase 1 encryption algorithm
Why it's wrong here
Phase 1 is already up, so its settings are fine; the error is in Phase 2.
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 NSE4 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 NSE4 exam.