- A
The web server's default gateway should be set to the firewall, not the Layer 3 switch
Why wrong: The web server's default gateway is the switch, which is correct for reaching other internal subnets; the firewall is for external traffic, but the server can still send reply to the firewall via the switch.
- B
The ACL on the DMZ SVI (VLAN 30) is blocking the return traffic from the web server to the firewall
The return traffic from the web server to the firewall must traverse the DMZ SVI. If the ACL on that SVI does not permit the firewall's IP or the traffic, it will be blocked.
- C
The ACL on the VLAN 10 SVI is blocking the traffic to the public IP because it only allows private IP ranges
Why wrong: The ACL permits any any after the deny, so it allows all traffic from VLAN 10 to any destination.
- D
The firewall is blocking the return traffic due to a stateful inspection rule
Why wrong: The firewall logs show the traffic is allowed, and the firewall should allow return traffic for established sessions.
Quick Answer
The answer is the ACL on the DMZ SVI (VLAN 30) blocking return traffic from the web server to the firewall. This occurs because of asymmetric routing on the Layer 3 switch: when users in VLAN 10 access the web server via the public IP, traffic enters the DMZ through the firewall, but the web server’s default gateway is the Layer 3 switch (192.168.30.1), so return traffic is sent to the switch instead of back through the firewall. The ACL applied inbound on the VLAN 30 SVI only permits traffic sourced from VLAN 10 to the DMZ, not the reverse path from the DMZ web server to the user’s IP, causing the return packets to be dropped. On the ISC2 Certified in Cybersecurity CC exam, this scenario tests your understanding of ACL direction and how asymmetric routing can bypass firewall state tables, a common trap where candidates focus only on the inbound ACL on the user VLAN. Remember: ACLs are directional, so always check the return path when traffic takes a different route back. Memory tip: “Traffic in, ACL on the out—check the reverse route to avoid the doubt.”
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 medium-sized company uses a network with three VLANs: VLAN 10 (Users, 192.168.10.0/24), VLAN 20 (Servers, 192.168.20.0/24), and VLAN 30 (DMZ, 192.168.30.0/24). A Layer 3 switch with an ACL is used for inter-VLAN routing. The company has a web server in the DMZ that must be accessible from the internet (via a public IP mapped to 192.168.30.10). Users in VLAN 10 need to access the web server on its private IP (192.168.30.10) for internal testing. The ACL is applied inbound on the VLAN 10 SVI. The ACL currently has the following entries: permit ip 192.168.10.0 0.0.0.255 192.168.30.0 0.0.0.255; deny ip any 192.168.20.0 0.0.0.255; permit ip any any. Recently, the security team noticed that users can access the web server on its private IP, but they cannot access the web server via the public IP (which goes through the firewall and then to the DMZ). The firewall logs show that traffic from the users to the public IP is allowed and reaches the DMZ web server, but the return traffic is blocked. The web server's default gateway is the Layer 3 switch (192.168.30.1). Which of the following is the most likely cause of the problem?
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 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 ACL on the DMZ SVI (VLAN 30) is blocking the return traffic from the web server to the firewall
The correct answer is B. The web server's default gateway is the Layer 3 switch (192.168.30.1). When users access the web server via the public IP, traffic enters the DMZ through the firewall, but the web server sends return traffic to its default gateway (the Layer 3 switch) instead of back to the firewall. The ACL applied inbound on the VLAN 30 SVI (the DMZ SVI) blocks this return traffic because the source is the web server (192.168.30.10) and the destination is the user's IP (192.168.10.x), which is not explicitly permitted by the ACL (the ACL only permits traffic from VLAN 10 to DMZ, not the reverse).
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 web server's default gateway should be set to the firewall, not the Layer 3 switch
Why it's wrong here
The web server's default gateway is the switch, which is correct for reaching other internal subnets; the firewall is for external traffic, but the server can still send reply to the firewall via the switch.
- ✓
The ACL on the DMZ SVI (VLAN 30) is blocking the return traffic from the web server to the firewall
Why this is correct
The return traffic from the web server to the firewall must traverse the DMZ SVI. If the ACL on that SVI does not permit the firewall's IP or the traffic, it will be blocked.
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 ACL on the VLAN 10 SVI is blocking the traffic to the public IP because it only allows private IP ranges
- ✗
The firewall is blocking the return traffic due to a stateful inspection rule
Why it's wrong here
The firewall logs show the traffic is allowed, and the firewall should allow return traffic for established sessions.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the concept of asymmetric routing and the importance of ACL directionality, where candidates mistakenly focus on the inbound ACL on the user VLAN (VLAN 10) instead of realizing that the return traffic is blocked by the ACL on the DMZ SVI (VLAN 30) due to the web server's default gateway pointing to the Layer 3 switch.
Trap categories for this question
Command / output trap
The firewall logs show the traffic is allowed, and the firewall should allow return traffic for established sessions.
Detailed technical explanation
How to think about this question
The problem illustrates asymmetric routing: traffic enters the DMZ via the firewall, but the web server's default gateway (the Layer 3 switch) sends return traffic through the switch, which applies an inbound ACL on the VLAN 30 SVI. Since the ACL only permits traffic from VLAN 10 to DMZ (not DMZ to VLAN 10), the return packets are denied. In a properly configured network, either the web server's default gateway should be the firewall, or the ACL must permit the return traffic (e.g., by adding a permit statement for established connections or specific source/destination pairs). This scenario is common when using a 'router-on-a-stick' or Layer 3 switch with ACLs and a separate firewall for internet traffic.
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 security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
What to study next
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FAQ
Questions learners often ask
What does this CC question test?
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 ACL on the DMZ SVI (VLAN 30) is blocking the return traffic from the web server to the firewall — The correct answer is B. The web server's default gateway is the Layer 3 switch (192.168.30.1). When users access the web server via the public IP, traffic enters the DMZ through the firewall, but the web server sends return traffic to its default gateway (the Layer 3 switch) instead of back to the firewall. The ACL applied inbound on the VLAN 30 SVI (the DMZ SVI) blocks this return traffic because the source is the web server (192.168.30.10) and the destination is the user's IP (192.168.10.x), which is not explicitly permitted by the ACL (the ACL only permits traffic from VLAN 10 to DMZ, not the reverse).
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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Last reviewed: Jun 30, 2026
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.
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