easyMultiple ChoiceObjective-mapped
ISC2 CC Practice Question: A network technician is setting up a remote…
A network technician is setting up a remote access VPN for employees using IPsec. The company's firewall is configured to allow IPsec traffic. Employees report that they can successfully establish the VPN connection (tunnel appears up), but they cannot ping or access any internal resources (e.g., file servers). The firewall logs show that packets from the VPN client IP addresses are being dropped at the firewall interface. Which of the following is the MOST likely cause of this issue?
⚠ Common exam trap
ISC2 often tests the distinction between tunnel establishment (IPsec Phase 1 and Phase 2) and traffic forwarding (ACL/permit rules), leading candidates to mistakenly blame encryption mismatches or client IP assignment when the real issue is a missing firewall rule.
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 firewall's access control list does not permit traffic from the VPN subnet to the internal network.
The VPN tunnel is established, meaning Phase 1 and Phase 2 of IPsec are complete and the client has a valid IP from the pool. However, packets from the VPN subnet are being dropped at the firewall interface, which indicates that the firewall's access control list (ACL) does not include a permit statement for traffic sourced from the VPN client subnet destined to the internal network. Without this ACL entry, the firewall will drop the traffic even though the tunnel is up.
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 VPN client is not assigned a correct IP address from the pool.
Why it's wrong here
Incorrect IP assignment would typically prevent tunnel establishment or routing.
- ✓
The firewall's access control list does not permit traffic from the VPN subnet to the internal network.
Why this is correct
The tunnel being up indicates IPsec negotiation succeeded, but the firewall still needs ACL to allow forwarded traffic.
- ✗
The firewall's intrusion prevention system is blocking the traffic.
Why it's wrong here
IPS may block specific malicious traffic, but logs show drops at firewall, not IPS; also, all traffic is dropped, not selective.
- ✗
The IPsec encryption algorithm is incompatible between client and firewall.
Why it's wrong here
If incompatible, the tunnel would not establish.
Visual reference
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
Access Control Fundamentals
Key term
Internet Protocol Security
Internet Protocol Security (IPsec) is a suite of protocols that encrypts and authenticates data packets sent over IP networks to ensure private and secure communication.
Key term
Internet Key Exchange
Internet Key Exchange (IKE) is a protocol used to set up a secure, encrypted connection between two devices by automatically negotiating and exchanging encryption keys over an untrusted network like the internet.
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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.