- A
The ACL is applied inbound on VLAN 100, so it only filters traffic entering VLAN 100, not traffic leaving VLAN 100.
Correct because inbound ACLs filter traffic entering the interface; traffic from VLAN 100 to VLAN 200 is leaving VLAN 100 and is not filtered.
- B
The ACL is applied outbound on VLAN 100, so it filters traffic leaving VLAN 100, preventing replies.
Why wrong: Incorrect because the scenario states the ACL is applied inbound.
- C
The ACL is applied to the wrong subinterface; it should be applied to the subinterface for VLAN 200.
Why wrong: Incorrect because applying the ACL to VLAN 100 is correct for filtering traffic to that VLAN.
- D
The ACL is blocking ICMP echo replies from VLAN 100 to VLAN 200.
Why wrong: Incorrect because the ACL permits traffic from VLAN 200 to VLAN 100, but does not affect traffic from VLAN 100 to VLAN 200.
350-401 ACLs and CoPP Practice Question
This 350-401 practice question tests your understanding of acls and copp. 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 engineer is troubleshooting an issue where a Cisco router is not forwarding traffic between two VLANs. The router has an ACL applied to the subinterface for VLAN 100 that permits traffic from VLAN 200 to VLAN 100, but denies all other traffic. Hosts in VLAN 200 can ping hosts in VLAN 100, but hosts in VLAN 100 cannot ping hosts in VLAN 200. The engineer checks the ACL and finds that it is applied inbound on the subinterface for VLAN 100. What is the most likely cause of the 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 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 is applied inbound on VLAN 100, so it only filters traffic entering VLAN 100, not traffic leaving VLAN 100.
The correct answer is that the ACL is applied inbound on VLAN 100, so it filters traffic entering VLAN 100; traffic from VLAN 200 to VLAN 100 is permitted, but traffic from VLAN 100 to VLAN 200 is not affected by this ACL. Option B is incorrect because the ACL is applied inbound, not outbound. Option C is incorrect because the ACL is applied to the subinterface, not the VLAN. Option D is incorrect because the ACL does not affect routing between VLANs; it only filters traffic.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 ACL is applied inbound on VLAN 100, so it only filters traffic entering VLAN 100, not traffic leaving VLAN 100.
Why this is correct
Correct because inbound ACLs filter traffic entering the interface; traffic from VLAN 100 to VLAN 200 is leaving VLAN 100 and is not filtered.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The ACL is applied outbound on VLAN 100, so it filters traffic leaving VLAN 100, preventing replies.
Why it's wrong here
Incorrect because the scenario states the ACL is applied inbound.
- ✗
The ACL is applied to the wrong subinterface; it should be applied to the subinterface for VLAN 200.
- ✗
The ACL is blocking ICMP echo replies from VLAN 100 to VLAN 200.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario states the ACL is applied inbound.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-401 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 350-401 question test?
ACLs and CoPP — This question tests ACLs and CoPP — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The ACL is applied inbound on VLAN 100, so it only filters traffic entering VLAN 100, not traffic leaving VLAN 100. — The correct answer is that the ACL is applied inbound on VLAN 100, so it filters traffic entering VLAN 100; traffic from VLAN 200 to VLAN 100 is permitted, but traffic from VLAN 100 to VLAN 200 is not affected by this ACL. Option B is incorrect because the ACL is applied inbound, not outbound. Option C is incorrect because the ACL is applied to the subinterface, not the VLAN. Option D is incorrect because the ACL does not affect routing between VLANs; it only filters traffic.
What should I do if I get this 350-401 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 350-401 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 18, 2026
This 350-401 practice question is part of Courseiva's free Cisco 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 350-401 exam.
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