Troubleshooting Phase 2 Proposal Mismatch in IPsec VPN
An administrator is troubleshooting an IPsec VPN Phase 2 negotiation failure. The debug shows 'no matching phase 2 proposal' from the remote peer. Which TWO of the following are likely causes? (Choose two.)
Quick Answer
The answer is that a Phase 2 proposal mismatch in IPsec VPN is most commonly caused by mismatched encryption or authentication algorithms and mismatched proxy IDs between the two peers. This occurs because Phase 2 negotiations use a proposal exchange where each peer sends its configured set of parameters, including encryption algorithms like AES128 or AES256, authentication hashes, and the specific local and remote subnets defined as proxy IDs; if any of these values do not align exactly with what the remote peer expects, the negotiation fails with the 'no matching proposal' error. On the Fortinet NSE 7 Advanced Security NSE7 exam, this question tests your ability to distinguish between Phase 2 configuration errors and unrelated issues like firewall policies or routing, which are common traps—remember that Phase 2 is about data-plane protection, not connectivity. A useful memory tip is to think of Phase 2 as a "double match": you need both the crypto suite and the traffic selectors (proxy IDs) to match, or the tunnel will not establish.
⚠ Common exam trap
Many exam-takers confuse Phase 1 and Phase 2 parameters, incorrectly attributing a Phase 2 'no matching proposal' error to authentication or IKE version mismatches, which actually cause Phase 1 failures.
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 local and remote proxy IDs (subnets) are not matching
IPsec Phase 2 negotiation requires the proxy IDs (local and remote subnets) to match exactly between peers. The 'no matching phase 2 proposal' debug message indicates the remote peer received a proposal with a subnet or traffic selector that does not match its configured proxy ID. This is a common misconfiguration when defining which traffic should be encrypted over the VPN tunnel.
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 local and remote proxy IDs (subnets) are not matching
Why this is correct
Phase 2 requires matching proxy IDs to establish SAs.
- ✗
The pre-shared key is incorrect
Why it's wrong here
PSK mismatch causes Phase 1 authentication failure, not Phase 2 proposal mismatch.
- ✗
The firewall policy does not allow UDP port 500
Why it's wrong here
This would affect Phase 1 negotiation, not Phase 2.
- ✓
The encryption algorithm (e.g., AES256 vs AES128) does not match between peers
Why this is correct
Phase 2 proposals must match for encryption and authentication algorithms.
- ✗
The IKE version (IKEv1 vs IKEv2) is different
Why it's wrong here
IKE version mismatch prevents Phase 1 from completing, not Phase 2.
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.
Go deeper
Related to this question
About these practice questions
Courseiva writes every NSE7 question from scratch — 940 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
Same concept, more angles
2 more ways 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. A site-to-site IPsec VPN tunnel is failing. The administrator runs 'diagnose vpn ike config' and sees that phase 1 parameters are correct. However, phase 2 negotiation fails with 'no proposal chosen'. What is the MOST likely cause?
medium- A.The pre-shared key is incorrect
- ✓ B.The phase 2 encryption/authentication algorithms do not match between peers
- C.The firewall policy allowing IKE traffic is missing
- D.The remote gateway IP address is wrong
Why B: The 'no proposal chosen' error in phase 2 indicates that the IPsec security association (SA) parameters—specifically the encryption algorithm, authentication algorithm, or Diffie-Hellman group—do not match between the two peers. Since the administrator confirmed phase 1 is correct via 'diagnose vpn ike config', the issue is isolated to phase 2 proposal mismatch, making option B the most likely cause.
Variation 2. 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?
medium- ✓ A.The Phase 1 proposal list does not include the algorithm combination the remote gateway requires
- B.The pre-shared key is incorrect
- C.The Phase 2 selectors are misconfigured
- D.The IKE version is set to 1 but remote uses 2
Why A: 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.
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.