NSE7 Troubleshooting and Diagnostics Practice Question
A network administrator is troubleshooting an IPsec VPN tunnel that fails to establish. The remote gateway logs show a proposal mismatch. On FortiGate, the administrator runs 'diagnose vpn ike config' and sees 'proposal: aes128-sha1, aes256-sha256'. The remote side expects 'aes256-sha1'. What is the most likely cause?
⚠ Common exam trap
Many candidates confuse a proposal mismatch with a pre-shared key or IKE version issue, but the specific error message and the 'diagnose vpn ike config' output directly point to an algorithm mismatch in Phase 1, not authentication or version negotiation.
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 Phase 1 proposal list does not include the algorithm combination the remote gateway requires
The 'diagnose vpn ike config' output shows the FortiGate's Phase 1 proposal list includes 'aes128-sha1' and 'aes256-sha256', but the remote gateway expects 'aes256-sha1'. Since neither of the local proposals matches the remote's required combination, the IKE negotiation fails with a 'proposal mismatch' error. The administrator must add 'aes256-sha1' to the Phase 1 proposal list on the FortiGate to align with the remote gateway's expectation.
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 Phase 1 proposal list does not include the algorithm combination the remote gateway requires
Why this is correct
Correct. The local proposal list must contain at least one matching algorithm set that the remote gateway supports.
- ✗
The pre-shared key is incorrect
Why it's wrong here
An incorrect pre-shared key would cause authentication failure, not proposal mismatch.
- ✗
The Phase 2 selectors are misconfigured
Why it's wrong here
Phase 2 selectors define which traffic is encrypted, not the encryption algorithm for IKE.
- ✗
The IKE version is set to 1 but remote uses 2
Why it's wrong here
The output shows IKE config, not version mismatch. A version mismatch would show different errors.
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
Related to this question
About these practice questions
One of 940 original NSE7 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
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.