Question 2,050 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that uRPF strict mode drops the packets because the switch lacks a specific route back to the host’s subnet via the ingress interface. In strict mode, the switch performs a FIB lookup on the source IP address of incoming packets—in this case, 2001:db8:10::100 arriving on the VLAN 20 interface—and requires an exact match route pointing back out the same interface. Since the switch only has a default route toward a next-hop router rather than a specific route for 2001:db8:10::/64 pointing to VLAN 10, the uRPF strict mode check fails, causing the drop. This scenario tests your understanding of how uRPF strict mode interacts with routing tables, a common trap on the Cisco CCNP ENARSI 300-410 exam where candidates mistakenly assume a default route satisfies the check. Remember: strict mode demands a specific route back to the source, not a default—think “strict needs specific, default gets dropped.”

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.

A network engineer is troubleshooting a connectivity issue where a host on VLAN 10 cannot reach a server on VLAN 20. Both VLANs are on the same switch, which is running IPv6. The engineer checks the switch and finds that uRPF (unicast Reverse Path Forwarding) is enabled in strict mode on the VLAN 20 interface. The host's IPv6 address is 2001:db8:10::100/64, and the server's address is 2001:db8:20::200/64. The switch has a default route pointing to a next-hop router. The host sends traffic to the server, but the switch drops the packets. What is the most likely cause?

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 uRPF strict mode check fails because the switch does not have a specific route to the host's subnet pointing back to the VLAN 10 interface.
Correct because uRPF strict mode requires a matching route in the FIB that points to the same interface on which the packet was received; a default route does not satisfy this requirement.
B
The uRPF mode should be loose mode to allow traffic from any source as long as there is a route in the FIB.
Why wrong: Incorrect because loose mode would still require a route to the source, but a default route would satisfy loose mode; however, the scenario indicates strict mode is the issue.
C
The host's IPv6 address is not in the switch's neighbor cache.
Why wrong: Incorrect because uRPF checks the routing table, not the neighbor cache.
D
The switch has an ACL that blocks traffic between VLANs.
Why wrong: Incorrect because the symptom is specifically related to uRPF, not an ACL.

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 uRPF strict mode check fails because the switch does not have a specific route to the host's subnet pointing back to the VLAN 10 interface.

With uRPF strict mode enabled on the VLAN 20 interface, the switch checks that the source IP address of incoming packets (2001:db8:10::100) is reachable via the same interface on which the packet arrived. Since the switch only has a default route pointing to a next-hop router and no specific route for the 2001:db8:10::/64 subnet pointing back to the VLAN 10 interface, the FIB lookup for the source address fails the strict uRPF check, causing the switch to drop the packet.

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 uRPF strict mode check fails because the switch does not have a specific route to the host's subnet pointing back to the VLAN 10 interface.
Why this is correct
Correct because uRPF strict mode requires a matching route in the FIB that points to the same interface on which the packet was received; a default route does not satisfy this requirement.
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 uRPF mode should be loose mode to allow traffic from any source as long as there is a route in the FIB.
Why it's wrong here
Incorrect because loose mode would still require a route to the source, but a default route would satisfy loose mode; however, the scenario indicates strict mode is the issue.
✗
The host's IPv6 address is not in the switch's neighbor cache.
Why it's wrong here
Incorrect because uRPF checks the routing table, not the neighbor cache.
✗
The switch has an ACL that blocks traffic between VLANs.
Why it's wrong here
Incorrect because the symptom is specifically related to uRPF, not an ACL.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between strict and loose uRPF modes, and the trap here is that candidates assume a default route is sufficient for strict mode, not realizing that strict mode requires a specific route pointing back to the source's ingress interface.

Trap categories for this question

Scenario analysis trap
Incorrect because loose mode would still require a route to the source, but a default route would satisfy loose mode; however, the scenario indicates strict mode is the issue.

Detailed technical explanation

How to think about this question

uRPF strict mode (defined in RFC 3704) performs a FIB lookup on the source address and verifies that the best reverse path route uses the same ingress interface. In IPv6, the FIB is populated by routing protocols or static routes; a default route alone does not satisfy strict mode unless it points back to the source's subnet interface. This behavior is critical in scenarios where asymmetric routing is present, as strict mode will drop traffic that does not follow a symmetric path.

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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.

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 uRPF strict mode check fails because the switch does not have a specific route to the host's subnet pointing back to the VLAN 10 interface. — With uRPF strict mode enabled on the VLAN 20 interface, the switch checks that the source IP address of incoming packets (2001:db8:10::100) is reachable via the same interface on which the packet arrived. Since the switch only has a default route pointing to a next-hop router and no specific route for the 2001:db8:10::/64 subnet pointing back to the VLAN 10 interface, the FIB lookup for the source address fails the strict uRPF check, causing the switch to drop the packet.

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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