300-410 VPN Technologies Practice Question
A network administrator is troubleshooting an IPsec VPN between two Cisco routers. Phase 1 completes successfully, but Phase 2 fails. The administrator sees the log message 'QM FSM error' on the initiator. Which configuration mismatch is the most likely cause?
⚠ Common exam trap
The trap here is assuming that any mismatch causes Phase 1 failure; QM FSM error specifically indicates a Phase 2 negotiation problem, so focus on transform sets and other Phase 2 parameters.
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 transform sets
A QM FSM error during Phase 2 typically points to a mismatch in the quick mode parameters, most commonly the transform set. The transform set defines the encryption and authentication algorithms for the IPsec SA. If the initiator proposes a transform set that the responder does not have configured or does not support, the responder may reject the proposal, causing the initiator to log a QM FSM error. Verifying that both peers have identical transform sets resolves the issue.
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 encryption algorithm in Phase 1
Why it's wrong here
If the Phase 1 encryption algorithms (e.g., AES vs. 3DES) do not match, Phase 1 will fail with a 'NO_PROPOSAL_CHOSEN' error. The fact that Phase 1 completed successfully means the Phase 1 encryption algorithm is agreed upon. Therefore, this is not the cause of a Phase 2 failure.
- ✗
Mismatched DH group in Phase 1
Why it's wrong here
A mismatch in the Diffie-Hellman group during Phase 1 would cause Phase 1 to fail, not Phase 2. Phase 1 would not complete if the DH groups were different. Since Phase 1 is successful, the DH group configuration is compatible, and the problem is specific to Phase 2 parameters.
- ✓
Mismatched transform sets
Why this is correct
The QM FSM error during quick mode often indicates that the responder cannot find a matching transform set for the Phase 2 proposal. If the initiator offers a transform set that the responder does not support or has not configured, the responder may send a delete or fail to process the quick mode exchange, leading to the QM FSM error on the initiator.
- ✗
Mismatched IKEv1 pre-shared keys
Why it's wrong here
A mismatched pre-shared key would cause Phase 1 to fail, typically with a 'pre-shared key mismatch' or 'NO_PROPOSAL_CHOSEN' error. Since Phase 1 completed, the pre-shared keys are correct. The QM FSM error occurs during quick mode (Phase 2), so the issue lies in Phase 2 parameters, not Phase 1 authentication.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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