156-315.81.20 Advanced VPN Design Practice Question
Exhibit
vpn debug ikeon [IKE] Peer 10.0.0.1:500 - Proxy ID mismatch [IKE] Phase 2 proposal rejected
Refer to the exhibit. What is the cause of the 'Proxy ID mismatch' error in the VPN debug output?
⚠ Common exam trap
Candidates often assume Proxy ID mismatches are related to routing or IKE Phase 1 keys. They fail to realize this is strictly a Phase 2 traffic selector negotiation issue between gateways.
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 traffic selectors (VPN domains) defined on both sides do not match.
A Proxy ID mismatch occurs when the traffic selectors defined in Phase 2 do not match between the two gateways. These selectors define which subnets are permitted to communicate through the tunnel. If one gateway expects a wider range of traffic or a different subnet mask than the other, the negotiation fails. This is a common configuration error in site-to-site VPNs involving mismatched VPN domain definitions or overlapping interest groups.
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 Diffie-Hellman group configuration is mismatched.
Why it's wrong here
A DH group mismatch causes a Phase 1 failure, not a Proxy ID mismatch. Proxy IDs refer specifically to the traffic selection (source and destination networks) during the Quick Mode/Phase 2 negotiation, which is a different layer of the IKE process than the cryptographic key exchange parameters.
- ✓
The traffic selectors (VPN domains) defined on both sides do not match.
Why this is correct
Proxy IDs are the encrypted subnets identified during the Phase 2 negotiation. If Gateway A expects traffic for 10.1.1.0/24 but Gateway B is only configured for 10.1.0.0/16, the IDs will not match, causing the negotiation to be rejected for security reasons to prevent traffic misrouting.
- ✗
The pre-shared secret key is invalid.
Why it's wrong here
An incorrect pre-shared secret would result in an authentication failure during Phase 1. It does not lead to a Proxy ID mismatch, which occurs during Phase 2 after authentication has already been successfully validated by both participating VPN gateways.
- ✗
The gateway has reached the maximum number of concurrent tunnels.
Why it's wrong here
Reaching a tunnel limit results in a 'resource exhausted' error message. The Proxy ID mismatch error specifically indicates a logical disagreement between the gateways regarding which traffic is authorized to traverse the tunnel, which is a policy configuration issue rather than a hardware resource limitation.
Visual reference
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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.