Question 1,117 of 2,015

ACLs and CoPP →easyMultiple ChoiceObjective-mapped

CCNP 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.

Question 1easymultiple choice

Open the full VLAN trunking answer →

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.

Full breakdown with real-world context →

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.
Why it's wrong here
Incorrect because applying the ACL to VLAN 100 is correct for filtering traffic to that VLAN.
✗
The ACL is blocking ICMP echo replies from VLAN 100 to VLAN 200.
Why it's wrong here
Incorrect because the ACL permits traffic from VLAN 200 to VLAN 100, but does not affect traffic 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?

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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