Question 1,101 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the ACL on the VLAN 20 subinterface is missing a permit entry for the return traffic, or the ACL was only applied inbound on the VLAN 10 subinterface. In a router-on-a-stick design, each VLAN uses a separate subinterface, and an inbound ACL on the VLAN 10 subinterface only filters traffic entering that specific subinterface from VLAN 10. Return traffic from VLAN 20 enters the router through the VLAN 20 subinterface, so it is never evaluated by the VLAN 10 ACL. This scenario directly tests your understanding of IPv6 ACL placement in a multi-VLAN environment for the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming a single ACL controls bidirectional flow. Remember the memory tip: "One subinterface, one direction—ACLs don't cross VLAN boundaries."

300-410 IPv6 Traffic Filtering and uRPF Practice Question

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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.

An engineer applies an IPv6 ACL to filter traffic between two VLANs on a switch using a router-on-a-stick configuration. The ACL is applied inbound on the subinterface. Traffic from VLAN 10 to VLAN 20 is permitted, but return traffic from VLAN 20 to VLAN 10 is dropped. Which is the most likely explanation?

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 1hardmultiple choice

Open the full VLAN trunking answer →

A
The ACL is applied only on the VLAN 10 subinterface, so return traffic from VLAN 20 is not filtered but the ACL on VLAN 10 drops it because the source address matches a deny entry.
Why wrong: Incorrect. Return traffic enters via the VLAN 20 subinterface, not VLAN 10, so the ACL on VLAN 10 does not see it.
B
The ACL on the VLAN 20 subinterface is missing a permit entry for the return traffic, or the ACL is applied outbound on VLAN 10, which does not affect incoming return traffic.
Correct. The return traffic must be permitted by the ACL on the VLAN 20 subinterface (inbound) or on the VLAN 10 subinterface (outbound). If missing, traffic is dropped.
C
The router has 'ipv6 unicast-routing' disabled, preventing inter-VLAN routing.
Why wrong: Incorrect. Inter-VLAN routing would fail completely, not just return traffic.
D
The ACL uses 'deny ipv6 any any' which blocks all traffic, but the permit statement for VLAN 10 to VLAN 20 is placed after the deny, causing it to be ignored.
Why wrong: Incorrect. ACLs are processed sequentially; if a permit is before the deny, it works. The issue is asymmetric application.

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 on the VLAN 20 subinterface is missing a permit entry for the return traffic, or the ACL is applied outbound on VLAN 10, which does not affect incoming return traffic.

In router-on-a-stick, each VLAN has a separate subinterface. If the ACL is applied inbound on the subinterface for VLAN 10, it filters traffic entering that subinterface from VLAN 10. Return traffic from VLAN 20 enters the subinterface for VLAN 20, not VLAN 10. The ACL on VLAN 10 subinterface does not affect traffic from VLAN 20. The issue is likely that the ACL on the VLAN 20 subinterface is missing or configured incorrectly, or the engineer applied the ACL only on one subinterface. The edge case: engineers often apply ACLs only on one side, forgetting that traffic is bidirectional and each subinterface needs its own ACL.

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 only on the VLAN 10 subinterface, so return traffic from VLAN 20 is not filtered but the ACL on VLAN 10 drops it because the source address matches a deny entry.
Why it's wrong here
Incorrect. Return traffic enters via the VLAN 20 subinterface, not VLAN 10, so the ACL on VLAN 10 does not see it.
✓
The ACL on the VLAN 20 subinterface is missing a permit entry for the return traffic, or the ACL is applied outbound on VLAN 10, which does not affect incoming return traffic.
Why this is correct
Correct. The return traffic must be permitted by the ACL on the VLAN 20 subinterface (inbound) or on the VLAN 10 subinterface (outbound). If missing, traffic is dropped.
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 router has 'ipv6 unicast-routing' disabled, preventing inter-VLAN routing.
Why it's wrong here
Incorrect. Inter-VLAN routing would fail completely, not just return traffic.
✗
The ACL uses 'deny ipv6 any any' which blocks all traffic, but the permit statement for VLAN 10 to VLAN 20 is placed after the deny, causing it to be ignored.
Why it's wrong here
Incorrect. ACLs are processed sequentially; if a permit is before the deny, it works. The issue is asymmetric application.

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.

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 300-410 questions on switching, trunking, and access-port configuration.

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FAQ

Questions learners often ask

What does this 300-410 question test?

IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Access ports place end devices into a single VLAN..

What is the correct answer to this question?

The correct answer is: The ACL on the VLAN 20 subinterface is missing a permit entry for the return traffic, or the ACL is applied outbound on VLAN 10, which does not affect incoming return traffic. — In router-on-a-stick, each VLAN has a separate subinterface. If the ACL is applied inbound on the subinterface for VLAN 10, it filters traffic entering that subinterface from VLAN 10. Return traffic from VLAN 20 enters the subinterface for VLAN 20, not VLAN 10. The ACL on VLAN 10 subinterface does not affect traffic from VLAN 20. The issue is likely that the ACL on the VLAN 20 subinterface is missing or configured incorrectly, or the engineer applied the ACL only on one subinterface. The edge case: engineers often apply ACLs only on one side, forgetting that traffic is bidirectional and each subinterface needs its own ACL.

What should I do if I get this 300-410 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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