NSE4 Authentication and VPN Practice Question
An administrator is troubleshooting an IPsec VPN that fails to establish Phase 2. The Phase 1 is up. The administrator runs 'diagnose vpn ike log' and sees the message 'no matching phase2 proposal found'. What is the MOST likely cause?
⚠ Common exam trap
Many exam-takers confuse Phase 1 and Phase 2 failures, assuming a Phase 2 error like 'no matching phase2 proposal' is caused by authentication or encryption mismatches, when it is specifically a proxy ID or traffic selector mismatch.
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 proxy ID mismatch
The message 'no matching phase2 proposal found' indicates that the IPsec security associations (SAs) proposed by the remote peer do not match the local Phase 2 configuration. Phase 2 uses proxy IDs (local/remote subnets and ports) to define which traffic should be encrypted. A mismatch in these proxy IDs, such as incorrect subnet definitions or protocol/port values, prevents the IKE negotiation from completing Phase 2, even though Phase 1 (which authenticates and establishes the IKE SA) is already up.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Pre-shared key mismatch
Why it's wrong here
A pre-shared key (PSK) mismatch would cause Phase 1 authentication to fail, producing an error like 'invalid identity' or 'authentication failed' before any Phase 2 exchange occurs. Since the problem is observed after Phase 1 is already up, the PSK must have matched. Therefore, a PSK mismatch is eliminated as a cause of the Phase 2 failure.
- ✗
IKE version mismatch (IKEv1 vs IKEv2)
Why it's wrong here
IKEv1 and IKEv2 are different protocols that are negotiated during Phase 1 via SA payloads. A version mismatch would prevent the peers from even completing IKE SA establishment. Given that Phase 1 is successfully up, both peers have agreed on the same IKE version, so this cannot be the reason the Phase 2 tunnel fails.
- ✗
Phase 1 encryption algorithm mismatch
Why it's wrong here
Phase 1 encryption algorithm mismatch (e.g., AES128 vs AES256) would cause a Phase 1 proposal mismatch, resulting in an IKE SA that never reaches active state. The tunneling process cannot proceed to Phase 2 if Phase 1's encryption algorithms are not mutually acceptable. Because Phase 1 is operational, the encryption algorithms are already aligned, ruling out this option.
- ✓
Phase 2 proxy ID mismatch
Why this is correct
Phase 2 proxy IDs define the exact local and remote networks that the IPsec tunnel will protect. A mismatch in these traffic selectors—for example, one side sends 10.0.0.0/24 while the other expects 192.168.0.0/24—causes the Quick Mode negotiation to fail even after a successful Phase 1. FortiGate logs often show 'no matching Phase 2 proposal' or 'receive proxy ID not acceptable.' This is the classic cause of a Phase 2 failure.
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.