156-315.81.20 Advanced VPN Design Practice Question
Exhibit
vpn debug ikeon vpn debug on vpn debug trunc IKE_AUTH: IDr mismatch. Expected: 10.0.0.1, Received: 172.16.0.1
Refer to the exhibit. An administrator is troubleshooting a failed IKEv2 tunnel. What is the cause of the failure?
⚠ Common exam trap
Candidates often misdiagnose identity mismatch errors as cryptographic algorithm failures, wasting time checking encryption proposals instead of peer name settings.
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 peer is sending an identity that is not recognized.
The error message 'IDr mismatch' indicates that the identity sent by the peer does not match the identity configured in the local gateway's VPN community settings. This is a common security feature in IKEv2 to prevent unauthorized peer access. The administrator must ensure that the ID configured in the gateway object matches the ID provided by the remote peer during the authentication phase.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
An incorrect shared secret was provided.
Why it's wrong here
If the shared secret were incorrect, the authentication would fail with an 'Authentication failed' error. An 'ID mismatch' error specifically points to the peer identity field, which is a different part of the IKEv2 handshake process, occurring before the shared secret validation is even completed.
- ✓
The peer is sending an identity that is not recognized.
Why this is correct
The IDr (Identity Responder) mismatch error confirms that the peer's identity is not matching what the local gateway has defined. This identity check is a security requirement in IKEv2. The administrator must update the peer's identity configuration to match the expected ID being sent by the peer.
- ✗
The IKEv2 proposal is incorrectly configured.
Why it's wrong here
A proposal mismatch would occur in the IKE_SA_INIT stage, not the IKE_AUTH stage where the identity exchange happens. Since the debug shows an IKE_AUTH error, the cryptographic proposal has already been successfully agreed upon, and the failure is isolated to the peer identity validation logic.
- ✗
The VPN tunnel interface (VTI) is down.
Why it's wrong here
If the VTI were down, the gateway wouldn't be able to process the IKE packets for that tunnel. The presence of the error in the debug logs confirms that the tunnel interfaces and routing are functioning correctly, and the issue is specifically a mismatch in the peer identification.
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
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