NSE7 Troubleshooting and Diagnostics Practice Question
An administrator is troubleshooting a scenario where IPSec VPN tunnels between two FortiGates are flapping. The logs show Phase 1 is up but Phase 2 fails with 'no proposal chosen'. The remote FortiGate has multiple Phase 2 selectors configured. What is the most likely cause?
⚠ Common exam trap
A common mix-up: candidates confuse Phase 1 and Phase 2 failures, assuming any 'no proposal chosen' error is due to Phase 1 misconfigurations like PSK or certificates, when in fact Phase 1 is already up, isolating the issue to Phase 2 proxy ID mismatches.
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
✓
Mismatched Phase 2 proxy IDs (local/remote subnets).
The 'no proposal chosen' error during Phase 2, despite Phase 1 being up, indicates a mismatch in the IPsec security association (SA) parameters. Since the remote FortiGate has multiple Phase 2 selectors configured, the most likely cause is that the local and remote proxy IDs (local and remote subnets) do not match any of the configured selectors. Phase 2 negotiation uses these proxy IDs to define which traffic should be encrypted; if they don't align, the IKE SA cannot be established.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Mismatched Phase 2 proxy IDs (local/remote subnets).
Why this is correct
The error 'no proposal chosen' is often due to mismatched proxy IDs in Phase 2.
- ✗
Mismatched pre-shared keys.
Why it's wrong here
Pre-shared key mismatch would cause Phase 1 to fail, not Phase 2.
- ✗
Dead Peer Detection (DPD) settings are too aggressive.
Why it's wrong here
DPD issues would cause the tunnel to drop after being established, not a Phase 2 failure.
- ✗
Certificate validation failure.
Why it's wrong here
Certificate issues would affect Phase 1 if certificate authentication is used, but logs show Phase 1 is up.
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.