easyMultiple Choice
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
A client without a pool address could not complete IKE, so the tunnel would never come up — yet it does, and the firewall already logs dropped packets sourced from valid VPN client IPs. Address assignment is what DHCP or an IPAM pool handles for internal LAN clients, not the cause of policy drops at the firewall interface.
- ✓
The firewall's access control list does not permit traffic from the VPN subnet to the internal network.
Why this is correct
The firewall's ACL lacks a rule permitting traffic from the VPN client subnet to internal resources, so packets are dropped at the interface despite the tunnel being up. This satisfies the stem's constraint: IPsec negotiation succeeds, but the separate policy governing post-decryption traffic flow is missing.
- ✗
The firewall's intrusion prevention system is blocking the traffic.
Why it's wrong here
An IPS blocking decapsulated traffic is conceivable, yet the stem gives no signature or alert evidence, and the drops occur at the firewall interface for VPN client addresses. IPS suits known exploit or anomaly signatures. The likelier cause is absent firewall rules or routing for the VPN client subnet.
- ✗
The IPsec encryption algorithm is incompatible between client and firewall.
Why it's wrong here
An encryption mismatch would prevent Phase 2 completion, so the tunnel would not come up; the stem states it does. Algorithm negotiation suits troubleshooting failed IKE or IPsec establishment. Dropped client packets after tunnel establishment point to missing internal routing or firewall policy for the VPN pool.
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
Network Security Components and Controls
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
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