The answer is that the firewall’s access control list does not permit traffic from the VPN subnet to the internal network. This is correct because while IPsec Phase 1 and Phase 2 have successfully completed—meaning the tunnel is up and the client has a valid IP from the pool—the firewall is still inspecting packets at the interface level. Even with IPsec traffic allowed, the firewall’s ACL must explicitly include a permit statement for traffic sourced from the VPN client subnet and destined for internal resources like file servers; without it, the firewall drops those packets, causing the “tunnel up, no access” symptom. On the ISC2 Certified in Cybersecurity CC exam, this scenario tests your understanding of how VPN connectivity and firewall policies interact—a common trap is assuming a successful tunnel guarantees resource access, when in fact ACLs are a separate enforcement layer. Remember the memory tip: “Tunnel up, ACL stuck—permit the subnet for the luck.”
ISC2 CC Network Security Practice Question
This CC practice question tests your understanding of network security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Trap categories for this question
Command / output trap
IPS may block specific malicious traffic, but logs show drops at firewall, not IPS; also, all traffic is dropped, not selective.
Detailed technical explanation
How to think about this question
In a typical IPsec remote-access VPN, the VPN gateway (often the firewall) assigns an IP from a pool to the client and creates a virtual tunnel interface. Even after the tunnel is up, traffic must still traverse the firewall's security policies or ACLs; the firewall treats the decrypted traffic as if it originated from the tunnel interface's subnet. A common oversight is forgetting to add a rule that permits traffic from the VPN pool (e.g., 10.10.10.0/24) to internal resources (e.g., 192.168.1.0/24), causing all pings and file access to be silently dropped.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A developer is choosing between AES-256 (symmetric) and RSA-2048 (asymmetric) for encrypting a large file that will be sent to a partner. Symmetric encryption is fast but requires key exchange; asymmetric is slower but solves the key distribution problem. A hybrid approach — encrypt the file with AES, encrypt the AES key with RSA — is standard. Questions like this test whether you understand when each approach applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CC question in full detail.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this CC question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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