- A
The router is configured with an ACL that blocks traffic from VLAN 10 to VLAN 20
Why wrong: While possible, an ACL is not mentioned. The more basic issue is that no inter-VLAN routing configuration exists at all.
- B
The router does not have a route to the VLAN 20 subnet
If the router has no directly connected interface for VLAN 20 (e.g., no subinterface or SVI), it does not have a route to that subnet. Without a route, packets from VLAN 10 to VLAN 20 are dropped.
- C
The switch port connecting the user to the network is configured with port security limiting MAC addresses
Why wrong: Port security would affect connectivity to all devices, not just those in another VLAN. The user can already communicate within VLAN 10.
- D
The file server has a firewall blocking ICMP ping requests
Why wrong: Even if ping fails, the user might still be able to access services. The question implies total communication failure, likely due to routing.
Quick Answer
The answer is that the router lacks a route to the VLAN 20 subnet. Even though the router has a subinterface connected to VLAN 20, inter-VLAN routing requires a connected route in the routing table for that specific subnet; without it, the router cannot forward packets from VLAN 10 to VLAN 20. The user can reach the default gateway and local devices because those are on the same VLAN, but the router drops traffic destined for VLAN 20 since it has no path. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding that a router must have a directly connected or static route for each VLAN subnet—a common trap is assuming a physical or logical connection alone is sufficient. Remember, an 'ip helper-address' only forwards DHCP broadcasts, not general traffic, so it does not create a route. Memory tip: “No route, no route—connected or static, or traffic won’t make it.”
N10-009 Network Troubleshooting Practice Question
This N10-009 practice question tests your understanding of network troubleshooting. 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 user on VLAN 10 reports that they cannot access a file server on VLAN 20. The user can ping the default gateway (interface on the router) and other devices within VLAN 10. The router has an 'ip helper-address' for DHCP but no static routes or dynamic routing configured for inter-VLAN routing. The file server is correctly configured with an IP address in VLAN 20. What is the most likely cause of the communication failure?
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 router does not have a route to the VLAN 20 subnet
The router has no route to the VLAN 20 subnet. Even though the router is directly connected to VLAN 20 via its subinterface, inter-VLAN routing requires the router to have a connected route in its routing table for that subnet. Without a route (either connected or static), the router cannot forward packets from VLAN 10 to VLAN 20. The 'ip helper-address' only forwards DHCP broadcasts, not general traffic.
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 router is configured with an ACL that blocks traffic from VLAN 10 to VLAN 20
- ✓
The router does not have a route to the VLAN 20 subnet
Why this is correct
If the router has no directly connected interface for VLAN 20 (e.g., no subinterface or SVI), it does not have a route to that subnet. Without a route, packets from VLAN 10 to VLAN 20 are dropped.
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 switch port connecting the user to the network is configured with port security limiting MAC addresses
Why it's wrong here
Port security would affect connectivity to all devices, not just those in another VLAN. The user can already communicate within VLAN 10.
- ✗
The file server has a firewall blocking ICMP ping requests
Why it's wrong here
Even if ping fails, the user might still be able to access services. The question implies total communication failure, likely due to routing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a router with an 'ip helper-address' can route between VLANs, when in fact it only forwards specific broadcast traffic and does not provide inter-VLAN routing without proper routing entries.
Detailed technical explanation
How to think about this question
For inter-VLAN routing on a router-on-a-stick or Layer 3 switch, each VLAN subnet must be present in the routing table as a directly connected route. Without a route to the destination subnet, the router drops the packet and may send an ICMP Destination Unreachable (Network Unreachable) message. The 'ip helper-address' command uses UDP forwarding (typically ports 67/68 for DHCP) and does not create any routing entries.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
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.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Troubleshooting — This question tests Network Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router does not have a route to the VLAN 20 subnet — The router has no route to the VLAN 20 subnet. Even though the router is directly connected to VLAN 20 via its subinterface, inter-VLAN routing requires the router to have a connected route in its routing table for that subnet. Without a route (either connected or static), the router cannot forward packets from VLAN 10 to VLAN 20. The 'ip helper-address' only forwards DHCP broadcasts, not general traffic.
What should I do if I get this N10-009 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 11, 2026
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.
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