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
Phase 1 negotiation selects a matching proposal from both peers' lists. The remote expects aes256-sha1, which is absent from the local list containing only aes128-sha1 and aes256-sha256, so no common combination exists and the tunnel fails.
- ✗
The pre-shared key is incorrect
Why it's wrong here
A wrong pre-shared key produces authentication failure after proposal negotiation, not a proposal mismatch naming specific encryption and hash algorithms. Verifying the key is tempting because it is a common IPsec misconfiguration, and would be correct when logs show authentication or invalid-key errors rather than algorithm disagreement.
- ✗
The Phase 2 selectors are misconfigured
Why it's wrong here
Phase 2 selectors govern which traffic is encrypted once Phase 1 completes; they cannot cause a Phase 1 proposal mismatch between aes128-sha1/aes256-sha256 and aes256-sha1. Checking selectors is tempting because they are a frequent tunnel fault, and would be correct when the tunnel establishes but traffic fails to pass.
- ✗
The IKE version is set to 1 but remote uses 2
Why it's wrong here
An IKE version mismatch would surface as version negotiation failure, not as the listed Phase 1 proposal sets differing in encryption and hash algorithms. Checking IKE version is tempting because mismatched versions do break tunnels, and would be correct when one peer offers IKEv1 and the other IKEv2.
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.