The answer is to review the ACL configuration on the router interface. This is the correct first step because an Access Control List applied to the interface connected to VLAN 10 acts as the gatekeeper for all traffic leaving that subnet; if a deny statement or missing permit entry blocks the traffic destined for VLAN 20, inter-VLAN routing will fail regardless of other settings like routing tables or trunk configurations. On the Systems Security Certified Practitioner SSCP exam, this scenario tests your ability to isolate access control issues before diving into more complex Layer 3 problems, and a common trap is to immediately check routing protocols or VLAN membership instead of the explicit filter. Remember the memory tip: ACLs are the first line of defense—when troubleshooting inter-VLAN connectivity, always check the filter before the path.
SSCP Network and Communications Security Practice Question
This SSCP practice question tests your understanding of network and communications 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 security analyst is troubleshooting a network issue where users on VLAN 10 cannot reach a server on VLAN 20. The router has an ACL applied to the interface connected to VLAN 10. Which step should the analyst take first to isolate the problem?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Review the ACL configuration on the router interface
The ACL applied to the router interface connected to VLAN 10 is the most likely cause of the connectivity issue, as it can explicitly permit or deny traffic from VLAN 10 to VLAN 20. Reviewing the ACL configuration first allows the analyst to quickly determine if the traffic is being blocked by a deny statement or missing permit entry, which is a common and immediate cause of such inter-VLAN routing failures. This step is the most efficient because it directly addresses the access control mechanism at the routing boundary.
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.
✓
Review the ACL configuration on the router interface
Why this is correct
The ACL on the router interface can block traffic between VLANs; this is the first logical step.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Run a traceroute from a user machine to the server
Why it's wrong here
Traceroute is useful but should be done after checking ACLs.
✗
Check the ARP table for the server's MAC address
Why it's wrong here
ARP is used to resolve MAC addresses, not to troubleshoot ACL issues.
✗
Verify the VLAN configuration on the switch
Why it's wrong here
VLAN configuration is important but the router's ACL is more likely the issue given the symptom.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the principle of starting with the most likely cause at the Layer 3/4 boundary (the ACL) rather than jumping to lower-layer troubleshooting like ARP or VLAN verification, which would be premature when an ACL is explicitly present.
Detailed technical explanation
How to think about this question
ACLs on Cisco routers are processed in sequential order, and each rule is evaluated until a match is found; an implicit 'deny any' at the end of the ACL will block all traffic not explicitly permitted. In a real-world scenario, a common mistake is placing the ACL in the wrong direction (e.g., inbound on VLAN 10 instead of outbound toward VLAN 20), which can be quickly identified by reviewing the ACL configuration and its interface assignment. The 'show access-lists' command also displays hit counts, allowing the analyst to see if the ACL is actually matching the 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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network and Communications Security — This question tests Network and Communications Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Review the ACL configuration on the router interface — The ACL applied to the router interface connected to VLAN 10 is the most likely cause of the connectivity issue, as it can explicitly permit or deny traffic from VLAN 10 to VLAN 20. Reviewing the ACL configuration first allows the analyst to quickly determine if the traffic is being blocked by a deny statement or missing permit entry, which is a common and immediate cause of such inter-VLAN routing failures. This step is the most efficient because it directly addresses the access control mechanism at the routing boundary.
What should I do if I get this SSCP 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: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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