IPsec VPN Phase 2 'No Matching Proposal'
A user reports that they cannot connect to a remote office via IPsec VPN. Phase 1 is up, but Phase 2 fails to establish. The administrator runs 'diagnose vpn ike log' and sees 'no matching phase2 proposal'. What should be checked?
Quick Answer
The correct answer is to check that the Phase 2 proposal settings—specifically encryption, authentication, and Perfect Forward Secrecy (PFS)—match on both peers. This is because the "no matching phase2 proposal" error in the IPsec VPN Phase 2 negotiation occurs when the FortiGate receives a proposal from the remote peer that does not align with its own configured Phase 2 selectors; since Phase 1 successfully established the IKE SA, the mismatch is isolated to the data-plane parameters like AES, SHA, and DH group. On the Fortinet NSE 7 Advanced Security NSE7 exam, this scenario tests your ability to troubleshoot IPsec VPNs by reading `diagnose vpn ike log` output, and a common trap is to mistakenly check Phase 1 settings or firewall policies when the log clearly points to Phase 2. Remember the memory tip: "Phase 1 is the handshake, Phase 2 is the data path—if the log says 'no matching proposal', compare your selectors byte by byte."
⚠ Common exam trap
Candidates often confuse Phase 1 and Phase 2 failures, assuming any 'no matching proposal' error relates to IKE parameters, when in fact Phase 1 being up isolates the issue to the IPsec SA negotiation in Phase 2.
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 2 proposal settings (encryption, authentication, PFS) match on both peers
The error 'no matching phase2 proposal' indicates that the IPsec peers cannot agree on the Phase 2 parameters (encryption, authentication, PFS group, lifetime). Since Phase 1 is up, IKE negotiation for the secure channel succeeded, but the subsequent IPsec SA negotiation fails because the proposed transforms do not match. Option D directly addresses this mismatch by requiring verification of the Phase 2 proposal settings on both peers.
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 firewall policies allow IKE traffic
Why it's wrong here
Phase 1 is already up, so IKE traffic is allowed.
- ✗
The local and remote subnet definitions are correct
Why it's wrong here
Incorrect subnets cause traffic not to be encrypted, but the error message specifically mentions proposal mismatch.
- ✗
The pre-shared key is correct
Why it's wrong here
Pre-shared key is used in Phase 1, not Phase 2.
- ✓
The Phase 2 proposal settings (encryption, authentication, PFS) match on both peers
Why this is correct
Mismatched Phase 2 proposals prevent the tunnel from establishing.
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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Same concept, more angles
1 more way this is tested on NSE7
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You are troubleshooting a VPN phase 2 negotiation failure. The logs show 'no proposal chosen'. What is the MOST likely cause?
hard- A.The remote gateway IP is incorrect
- B.The pre-shared key mismatch
- C.The IKE version mismatch
- ✓ D.The phase 2 proposal settings differ between the peers
Why D: The 'no proposal chosen' error in VPN phase 2 indicates that the IPsec peers could not agree on a common set of phase 2 parameters (such as encryption algorithm, authentication algorithm, or PFS group). Since phase 2 negotiation occurs after IKE phase 1 has successfully completed, the issue is specifically with the IPsec SA proposal settings, not with pre-shared keys or IKE version. Therefore, differing phase 2 proposals between the peers are the most likely cause.
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.