156-315.81.20 Advanced VPN Design Practice Question
Exhibit
vpn debug ikeon [IKE] Peer 192.168.1.50:500 - No proposal found matching local configuration [IKE] Peer 192.168.1.50:500 - Error: Phase 1 failure
Refer to the exhibit. An administrator is troubleshooting a site-to-site VPN tunnel. What is the most effective next step to resolve the 'No proposal found' error?
⚠ Common exam trap
Candidates often assume the solution is to check the firewall policy or routing. However, 'No proposal found' is specifically an IKE Phase 1/2 mismatch issue that requires adjusting community parameters, not general policy.
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
✓
Update the VPN Community to match the peer's IKE parameters.
The 'No proposal found' error signifies that the Security Gateway does not recognize or accept the parameters offered by the remote peer. This is a common issue in large-scale VPN deployments where policy mismatches occur due to manual configuration errors. Synchronizing the IKE Phase 1 settings, specifically ensuring the Diffie-Hellman group and encryption standards align exactly, resolves the incompatibility and allows the tunnel negotiation to proceed.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Re-generate the internal CA certificate on the gateway.
Why it's wrong here
Re-generating the internal CA affects identity verification and trust, not the IKE proposal negotiation. If the peer is not recognized, the gateway will not even reach the certificate exchange stage. This action disrupts current VPN tunnels and is unnecessary for fixing an IKE negotiation proposal mismatch.
- ✗
Increase the IKE lifetime settings in the VPN Community.
Why it's wrong here
Lifetime settings are secondary to the primary cryptographic proposal. Increasing the lifetime will not resolve a mismatch error if the algorithms themselves (AES, SHA, DH) are configured differently. The peer needs to be configured to match the gateway's mandatory proposal set for a successful handshake.
- ✓
Update the VPN Community to match the peer's IKE parameters.
Why this is correct
This is the direct fix for a proposal mismatch. The Security Gateway must be configured to permit the specific encryption, authentication, and DH group settings that the remote peer is sending. Once the Community is updated to accept these parameters, the tunnel negotiation will successfully complete.
- ✗
Disable NAT-T on the Security Gateway interface.
Why it's wrong here
NAT-T is only relevant if there is a NAT device between the peers causing IKE header modification. Disabling it would cause failure if NAT-T is actually needed. It does not resolve an algorithmic proposal mismatch, which occurs regardless of the presence of NAT in the network path.
Visual reference
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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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
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