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ISC2 CC Practice Question: A medium-sized company uses a network with three…
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?
⚠ Common exam trap
ISC2 often tests asymmetric routing and the need for DMZ servers to use the firewall as their default gateway. Candidates may focus on the VLAN 10 ACL or assume a DMZ-side ACL, but the stated cause is the server's default gateway pointing to the L3 switch.
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 web server's default gateway should be set to the firewall, not the Layer 3 switch
The most likely cause is that the DMZ web server's default gateway is misconfigured. Because it points to the Layer 3 switch instead of the firewall, replies to public-IP requests are routed toward the L3 switch rather than back through the firewall. This breaks the firewall's stateful inspection/NAT path, so the return traffic is blocked or never completes. Private-IP access works because the L3 switch can route directly to VLAN 10. The VLAN 10 ACL is inbound and does not block the outbound request; no DMZ SVI ACL is described.
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 this is correct
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 it's wrong here
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.
- ✗
The ACL on the VLAN 10 SVI is blocking the traffic to the public IP because it only allows private IP ranges
Why it's wrong here
The ACL permits any any after the deny, so it allows all traffic from VLAN 10 to any destination.
- ✗
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.
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Related to this question
Learn chapter
BC/DR Planning and Testing
Key term
Access Control List
An Access Control List is a set of rules that decides which traffic is allowed or denied entry to a network or device.
Key term
ACL
An Access Control List is a set of rules that determines who or what can access specific network resources or data.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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