156-215.81.20 VPN Basics Practice Question
Exhibit
vpn debug mon [IKE] Peer 192.168.1.50: Phase 2 failure [IKE] Error: Proxy ID mismatch
Refer to the exhibit. A site-to-site VPN tunnel fails to initialize. What is the most likely cause of this error?
⚠ Common exam trap
Candidates often guess 'wrong shared secret' when a tunnel fails to initialize, ignoring that encryption domain mismatches are the most common source of Phase 2 negotiation failures.
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 encryption domain settings do not match
A Proxy ID mismatch in Phase 2 indicates that the two gateways have different ideas of what traffic should be protected by the tunnel. Proxy IDs are the traffic selectors that define the source and destination networks. If the gateways do not agree on these parameters, they will be unable to generate the matching security associations required to tunnel the traffic, leading to a negotiation failure.
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 pre-shared key is incorrect
Why it's wrong here
A PSK mismatch occurs in Phase 1, which happens before Phase 2. Since the error specifically states 'Phase 2 failure' and 'Proxy ID mismatch', the authentication phase was already successful, meaning the PSK is correct on both peers.
- ✓
The encryption domain settings do not match
Why this is correct
Proxy IDs are essentially the traffic selectors derived from the encryption domain. If the configured encryption domains are not identical or at least compatible between both gateways, they will propose different traffic selectors, causing the mismatch error in the Phase 2 handshake.
- ✗
The IKE version is mismatched
Why it's wrong here
While IKE versions can cause negotiation failures, they typically result in global proposal rejection or timeout messages. The specific 'Proxy ID' error points directly to a disagreement regarding the networks to be protected, not the IKE protocol version itself.
- ✗
The gateway is not authorized to peer
Why it's wrong here
If a gateway were not authorized to peer, the connection would be rejected at the start of the handshake. The error here specifically identifies the traffic selection phase as the point of failure, not the initial authorization or access control check.
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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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