Question 1,101 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

Troubleshooting IPv6 ACL on Subinterface: Return Traffic Dropped

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.

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 →

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."

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.

Option B is correct because in a router-on-a-stick configuration, traffic from VLAN 10 to VLAN 20 is permitted by the inbound ACL on the VLAN 10 subinterface. However, return traffic from VLAN 20 to VLAN 10 must traverse the VLAN 20 subinterface inbound (or the VLAN 10 subinterface outbound). If the ACL is applied inbound only on the VLAN 10 subinterface, return traffic from VLAN 20 is not inspected unless an ACL is also applied inbound on the VLAN 20 subinterface or outbound on the VLAN 10 subinterface. The most likely cause is that 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.

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 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
Read the scenario before looking for a memorised answer.
✗
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: answer the scenario, not the keyword

Cisco often tests the misconception that an ACL applied inbound on one subinterface controls all traffic between VLANs, when in fact it only filters traffic entering that specific subinterface, and return traffic must be permitted by an ACL on the opposite subinterface or by an outbound ACL.

Detailed technical explanation

How to think about this question

In a router-on-a-stick topology, each VLAN subinterface acts as a separate Layer 3 interface. Inbound ACLs filter traffic entering that subinterface from the VLAN, while outbound ACLs filter traffic leaving the subinterface toward the VLAN. For bidirectional communication, ACLs must be applied on both subinterfaces (or use reflexive/stateful filtering) to permit return traffic. A common mistake is to apply an ACL only on one subinterface, assuming it controls all traffic, but it only affects traffic entering that specific subinterface. In real-world deployments, this often leads to asymmetric traffic drops that are difficult to troubleshoot without understanding the directionality of ACL application.

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.

Visual reference

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 300-410 question test?

IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..

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. — Option B is correct because in a router-on-a-stick configuration, traffic from VLAN 10 to VLAN 20 is permitted by the inbound ACL on the VLAN 10 subinterface. However, return traffic from VLAN 20 to VLAN 10 must traverse the VLAN 20 subinterface inbound (or the VLAN 10 subinterface outbound). If the ACL is applied inbound only on the VLAN 10 subinterface, return traffic from VLAN 20 is not inspected unless an ACL is also applied inbound on the VLAN 20 subinterface or outbound on the VLAN 10 subinterface. The most likely cause is that 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.

What should I do if I get this 300-410 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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