Why IPsec VPN Phase 2 Fails with Proposal Mismatch
A network administrator has configured an IPsec VPN between two FortiGate devices. The Phase 1 proposal includes AES256-SHA256-DH14. The Phase 2 proposal includes AES128-SHA1. The VPN tunnel fails to establish. Which of the following is the MOST likely cause?
Quick Answer
The answer is a Phase 2 proposal mismatch between the two FortiGate devices. In IKEv1, unlike Phase 1 which can negotiate a common proposal from a list, Phase 2 requires both sides to have an identical encryption and authentication algorithm set; if one side offers AES128-SHA1 and the other expects AES256-SHA256, the tunnel simply fails to establish. This scenario tests your understanding of IPsec VPN Phase 2 proposal mismatch on the Fortinet NSE 4 exam, where a common trap is confusing Phase 1’s flexible negotiation with Phase 2’s strict exact-match requirement. Remember the memory tip: “Phase 1 can play, Phase 2 must stay”—meaning Phase 1 proposals can be negotiated, but Phase 2 proposals must be identical on both ends.
⚠ Common exam trap
Many candidates assume Phase 1 and Phase 2 proposals must be identical in strength or that a strong Phase 1 automatically implies a strong Phase 2, but NSE4 tests that Phase 2 proposals are independently negotiated and must match exactly between peers.
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 does not match between the two devices
The Phase 2 proposal must match exactly between both VPN endpoints for IPsec to establish. Here, the local device uses AES128-SHA1, but the remote device likely expects a different combination (e.g., AES256-SHA256 or another algorithm), causing the IKE negotiation to fail after Phase 1 completes. Even though Phase 1 succeeded with AES256-SHA256-DH14, the mismatch in Phase 2 prevents the IPsec SA from being created.
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 is too strong and the remote FortiGate does not support DH14
Why it's wrong here
If the remote side does not support DH14, Phase 1 would fail, but typically both sides are configured identically. The issue is Phase 2 mismatch.
- ✓
The Phase 2 proposal does not match between the two devices
Why this is correct
Phase 2 parameters must be identical on both sides. One side may have AES256 or a different lifetime, causing mismatch.
- ✗
The VPN policy has not been configured on the remote FortiGate
Why it's wrong here
Missing policy would cause traffic not to be encrypted, but the tunnel can still establish.
- ✗
The pre-shared key is incorrect
Why it's wrong here
Incorrect PSK would cause Phase 1 failure, not specifically Phase 2 mismatch.
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
3 more ways this is tested on NSE4
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 network administrator configured an IPsec VPN between two FortiGates. Phase 1 is up, but Phase 2 fails to establish. The diagnose output shows 'no matching proposal'. What is the MOST likely cause?
medium- A.The firewall policy allowing the VPN traffic is missing
- ✓ B.The Phase 2 encryption and authentication algorithms do not match between peers
- C.The pre-shared keys do not match
- D.The remote gateway IP address is incorrect
Why B: The 'no matching proposal' 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 FortiGate peers. Phase 2 uses these proposals to negotiate the IPsec SA for protecting data traffic, and a mismatch prevents the tunnel from establishing. Since Phase 1 completed successfully, the pre-shared keys and gateway IP are already verified, isolating the issue to Phase 2 configuration.
Variation 2. A network administrator configures an IPsec VPN between two FortiGate devices. Phase 1 completes successfully, but Phase 2 fails to establish. The administrator runs 'diagnose vpn ike log' and sees the error 'proposal mismatch'. What is the MOST likely cause?
medium- A.The IKE version is mismatched (IKEv1 vs IKEv2)
- B.The pre-shared key is incorrect
- C.The firewall policies are blocking IKE traffic on UDP port 500
- ✓ D.The Phase 2 local and remote subnets do not match on both ends
Why D: The error 'proposal mismatch' in the Phase 2 IKE log indicates that the IPsec security associations (SAs) proposed by one FortiGate do not match the configured Phase 2 parameters on the other. Since Phase 1 completed successfully, the IKE version and pre-shared key are already validated. The mismatch specifically refers to the local and remote subnet definitions, encryption algorithms, or authentication methods in the Phase 2 selectors. Therefore, the most likely cause is that the Phase 2 local and remote subnets are not correctly mirrored on both ends.
Variation 3. A network administrator is troubleshooting an IPsec VPN tunnel between two FortiGates. Phase 1 is up, but Phase 2 fails to establish. The debug command 'diagnose vpn ike log' shows: 'no suitable proposal found'. What is the most likely cause?
medium- ✓ A.Phase 2 encryption or authentication algorithms do not match on both sides.
- B.The firewall policy allowing IPsec traffic is missing.
- C.The remote gateway IP address is unreachable.
- D.The pre-shared key is incorrect.
Why A: The 'no suitable proposal found' error in Phase 2 of an IPsec VPN tunnel indicates that the Phase 2 parameters (encryption algorithm, authentication algorithm, or PFS settings) do not match between the two FortiGate peers. Since Phase 1 is up, the IKE SA is established, meaning pre-shared keys, remote gateway reachability, and basic firewall policies for IKE traffic are correct. The mismatch specifically occurs in the Phase 2 proposal negotiation, where each side sends its supported transforms and the responder cannot find a common set.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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