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ISC2 CC Practice Question: A company's remote access VPN uses IPsec with…
A company's remote access VPN uses IPsec with pre-shared keys. Employees report that they cannot connect from home. The VPN server logs show 'IKE authentication failed.' The help desk confirms the pre-shared keys are correct. Which of the following is the most likely cause?
⚠ Common exam trap
ISC2 often tests the distinction between 'IKE authentication failed' (which points to phase 1 parameter mismatch or PSK error) and 'no response' (which points to firewall/NAT blocking), trapping candidates who assume NAT is always the culprit without reading the log message carefully.
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 IKE phase 1 parameters (encryption, hash, DH group) do not match
'IKE authentication failed' specifically indicates a failure during IKE phase 1, where the peers negotiate security parameters. Even if the pre-shared key is correct, mismatched encryption (e.g., AES vs. 3DES), hash (e.g., SHA-1 vs. SHA-256), or Diffie-Hellman group (e.g., group 2 vs. group 14) will cause the IKE SA to fail, preventing phase 1 authentication from completing.
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 client is behind a NAT device that blocks IPsec traffic
Why it's wrong here
NAT traversal (NAT-T) is common and would not cause 'IKE authentication failed' if configured correctly.
- ✗
The VPN server is not responding to IKE requests
Why it's wrong here
If unresponsive, the error would be timeout, not authentication failure.
- ✗
The client's certificate has expired
Why it's wrong here
Using pre-shared keys, not certificates.
- ✓
The IKE phase 1 parameters (encryption, hash, DH group) do not match
Why this is correct
Mismatched parameters cause authentication failure despite correct keys.
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.
Go deeper
Related to this question
Learn chapter
Introduction to Security Principles
Key term
Group
A group is a collection of users, devices, or other objects that are assigned permissions and policies together for simplified management in identity and governance systems like Microsoft Entra ID.
Key term
VPN
A VPN creates an encrypted tunnel over a public network to securely connect remote users or sites to a private network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CC practice question is part of Courseiva's free ISC2 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 CC exam.