NSE7 Troubleshooting and Diagnostics Practice Question
A FortiGate VPN tunnel shows 'phase1 negotiation failed' in the logs. The remote gateway is a third-party device. The debug command 'diagnose vpn ike config' shows mismatched proposals. Which setting is MOST likely incorrect on the FortiGate?
⚠ Common exam trap
Many exam-takers confuse 'mismatched proposals' with authentication failures (pre-shared key) or identification issues (local ID), but the debug command specifically shows the proposal attributes, making encryption algorithm the most likely culprit.
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 encryption algorithm (e.g., AES256 vs 3DES)
The 'diagnose vpn ike config' command displays the IKE proposal parameters (encryption, authentication, DH group) that the FortiGate is configured to offer. When the log shows 'phase1 negotiation failed' and the debug output indicates 'mismatched proposals', it means the FortiGate's configured encryption algorithm (e.g., AES256) does not match any algorithm supported by the remote third-party device (e.g., 3DES). This is the most direct cause of proposal mismatch, as IKE phase 1 requires both sides to agree on a common transform set.
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 pre-shared key
Why it's wrong here
A pre-shared key mismatch produces an authentication failure in phase 1, but the debug output explicitly shows mismatched proposals, which is a parameter mismatch rather than an authentication error. PSK errors suit scenarios where proposals align yet the peer rejects authentication.
- ✗
The local ID type
Why it's wrong here
Local ID type affects peer identity verification during phase 1 authentication, not proposal matching. A local ID mismatch typically surfaces as an authentication or identity failure after proposals are accepted; the debug output here points to mismatched encryption, hash, or DH proposals instead.
- ✓
The encryption algorithm (e.g., AES256 vs 3DES)
Why this is correct
Phase 1 negotiation fails when the two peers' IKE proposals do not intersect. A mismatched encryption algorithm, such as AES256 against 3DES, means no common transform is offered, so the FortiGate's phase 1 proposal must be aligned with the third-party gateway's supported encryption.
- ✗
The DPD configuration
Why it's wrong here
DPD is for keepalive, not negotiation.
Quick reference
Symmetric Encryption Algorithm Comparison
| Algorithm | Key Size | Block Size | Status | Notes |
|---|---|---|---|---|
| AES-128 | 128-bit | 128-bit | Current standard | NIST approved; WPA3, TLS |
| AES-256 | 256-bit | 128-bit | Current standard | Preferred for sensitive / govt data |
| 3DES | 112-bit effective | 64-bit | Deprecated (2023) | Replaced by AES |
| DES | 56-bit | 64-bit | Broken | Cracked in < 24 h; never deploy |
| ChaCha20 | 256-bit | Stream cipher | Current | TLS 1.3, WireGuard |
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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.