Question 1,651 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple SelectObjective-mapped

IPv6 uRPF and ACL Interaction

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.

Which TWO statements about the interaction between IPv6 ACLs and uRPF are correct? (Choose TWO.)

A
When both uRPF and an inbound IPv6 ACL are configured on the same interface, uRPF is processed first.
Correct. uRPF checks occur before ACL processing in the input path.
B
An IPv6 ACL can be used to permit traffic that would otherwise be dropped by uRPF strict mode due to asymmetric routing.
Correct. You can use an ACL to explicitly permit traffic from sources that are not reachable via the incoming interface, bypassing uRPF drops.
C
uRPF can be configured to ignore IPv6 ACLs on the same interface.
Why wrong: Incorrect. uRPF and ACLs are independent; uRPF does not ignore ACLs. They operate sequentially.
D
An outbound IPv6 ACL can be used to filter traffic before uRPF checks.
Why wrong: Incorrect. uRPF checks occur on inbound traffic only; outbound ACLs are processed after routing, not before uRPF.
E
If uRPF drops a packet, the inbound ACL is still evaluated for logging purposes.
Why wrong: Incorrect. If uRPF drops a packet, it is discarded before reaching the ACL, so the ACL is not evaluated.

Full breakdown with real-world context →

Quick Answer

The answer is that an IPv6 ACL can be used to permit traffic that would otherwise be dropped by uRPF strict mode due to asymmetric routing. This is correct because on a Cisco router, uRPF processes packets before inbound ACLs in the input path; if uRPF drops a packet, the ACL is never evaluated, but if uRPF passes the packet, the ACL then filters it sequentially. For the CCNP ENARSI 300-410 exam, this interaction tests your understanding of packet flow order and the fact that ACLs can override uRPF drops only when explicitly configured to permit the traffic, which is a common trap—many candidates mistakenly think uRPF always overrides ACLs. Remember the memory tip: “uRPF first, ACL second; to save a packet from uRPF, the ACL must be beckoned.”

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

✓

When both uRPF and an inbound IPv6 ACL are configured on the same interface, uRPF is processed first.

Option A is correct because when both uRPF and an inbound IPv6 ACL are configured on the same interface, Cisco IOS processes uRPF first. This is because uRPF is a routing-based security check that verifies the source address against the FIB before any ACL filtering occurs. The inbound ACL is evaluated only after the packet passes the uRPF check, ensuring that spoofed traffic is dropped before ACL processing.

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.

✓
When both uRPF and an inbound IPv6 ACL are configured on the same interface, uRPF is processed first.
Why this is correct
Correct. uRPF checks occur before ACL processing in the input path.
Related concept
Read the scenario before looking for a memorised answer.
✓
An IPv6 ACL can be used to permit traffic that would otherwise be dropped by uRPF strict mode due to asymmetric routing.
Why this is correct
Correct. You can use an ACL to explicitly permit traffic from sources that are not reachable via the incoming interface, bypassing uRPF drops.
Related concept
Read the scenario before looking for a memorised answer.
✗
uRPF can be configured to ignore IPv6 ACLs on the same interface.
Why it's wrong here
Incorrect. uRPF and ACLs are independent; uRPF does not ignore ACLs. They operate sequentially.
✗
An outbound IPv6 ACL can be used to filter traffic before uRPF checks.
Why it's wrong here
Incorrect. uRPF checks occur on inbound traffic only; outbound ACLs are processed after routing, not before uRPF.
✗
If uRPF drops a packet, the inbound ACL is still evaluated for logging purposes.
Why it's wrong here
Incorrect. If uRPF drops a packet, it is discarded before reaching the ACL, so the ACL is not evaluated.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume ACLs are always processed before uRPF, or that uRPF can be configured to skip ACL checks, but Cisco explicitly tests the order of operations where uRPF is evaluated first on inbound interfaces.

Detailed technical explanation

How to think about this question

Under the hood, uRPF strict mode performs a FIB lookup on the source IP address of the incoming packet; if the best return route does not point to the ingress interface, the packet is dropped. When an IPv6 ACL is also configured, the order of operations is: uRPF check first, then inbound ACL, then routing, then outbound ACL. This sequence is critical for security, as it prevents spoofed packets from triggering ACL logging or consuming CPU resources.

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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

Visual reference

What to study next

Got this wrong? Here's your next step.

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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: When both uRPF and an inbound IPv6 ACL are configured on the same interface, uRPF is processed first. — Option A is correct because when both uRPF and an inbound IPv6 ACL are configured on the same interface, Cisco IOS processes uRPF first. This is because uRPF is a routing-based security check that verifies the source address against the FIB before any ACL filtering occurs. The inbound ACL is evaluated only after the packet passes the uRPF check, ensuring that spoofed traffic is dropped before ACL processing.

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.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

About these practice questions

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. Consider the following configuration: ipv6 access-list FILTER permit ipv6 2001:db8:3::/48 any deny ipv6 any any interface GigabitEthernet0/5 ipv6 traffic-filter FILTER in ipv6 verify unicast source reachable-via rx A packet arrives on GigabitEthernet0/5 with source 2001:db8:3::100 and destination 2001:db8:4::1. The route for 2001:db8:3::/48 points out interface GigabitEthernet0/6. What happens?

medium

A.The packet is permitted because the ACL matches and uRPF is not applied.
✓ B.The packet is dropped by uRPF because strict mode requires the source to be reachable via the receiving interface.
C.The packet is dropped by the ACL because the deny statement blocks all traffic.
D.The packet is permitted because uRPF only checks destination addresses.

Why B: The packet is dropped by uRPF (unicast Reverse Path Forwarding) in strict mode because the `ipv6 verify unicast source reachable-via rx` command checks that the source address 2001:db8:3::100 is reachable via the receiving interface (GigabitEthernet0/5). The route for the source prefix 2001:db8:3::48 points out GigabitEthernet0/6, not the receiving interface, so uRPF fails and drops the packet before the ACL is evaluated.

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