156-315.81.20 Advanced VPN Design Practice Question
Exhibit
vpn debug ikeon vpn debug on vpn debug trunc IKE_SA_init: Received IKE_SA_INIT request from 192.168.10.1 IKE_SA_init: Proposal mismatch, no common transform found.
Refer to the exhibit. An administrator is troubleshooting a VPN tunnel that fails to initialize. Based on the debug output, what is the most likely cause?
⚠ Common exam trap
Candidates often assume that Phase 1/IKE proposal mismatches are caused by pre-shared key typos, overlooking differing cryptographic algorithm selections 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 cryptographic settings between the peers are incompatible.
The debug indicates a proposal mismatch during the IKE_SA_INIT phase. This means the two gateways cannot agree on a common set of cryptographic algorithms. This is a common issue in multi-vendor VPNs where default settings differ. The administrator must verify the encryption, integrity, and Diffie-Hellman group settings on both ends to ensure they exactly match the configured proposal requirements for IKEv2.
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 peer IP address is incorrect in the VPN community.
Why it's wrong here
While an incorrect peer IP would lead to a failure, the debug explicitly states a 'Proposal mismatch.' This error confirms that the connectivity to the peer exists, but the security parameters requested by the initiator are being rejected by the responder due to cryptographic configuration differences.
- ✓
The cryptographic settings between the peers are incompatible.
Why this is correct
The error message 'Proposal mismatch' directly indicates that the Security Gateway and the peer cannot agree on the Phase 1 security parameters. This happens when encryption algorithms, hash functions, or DH groups do not align, causing the gateway to reject the incoming connection request to maintain security.
- ✗
The pre-shared secret is mismatched between the gateways.
Why it's wrong here
Pre-shared secret mismatches typically occur during the IKE_AUTH phase, not the IKE_SA_INIT phase. During SA_INIT, the peers negotiate security associations and cryptographic suites. Since the error occurs at the init stage, the authentication secret hasn't even been exchanged or validated yet by the gateway processes.
- ✗
The VPN tunnel interface (VTI) is configured with the wrong IP.
Why it's wrong here
VTI configuration issues usually result in routing failures rather than IKE phase 1 proposal mismatches. Since the gateway is actively processing the IKE packet and identifying a discrepancy in the proposal, the issue is purely cryptographic in nature and not related to the logical tunnel interface's IP settings.
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 Check Point exam blueprint
This 156-315.81.20 practice question is part of Courseiva's free Check Point 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 156-315.81.20 exam.