Question 146 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

uRPF Loose Mode Drops Traffic with Equal-Cost Paths — Link-Local Source

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 configures uRPF loose mode on an interface that is part of an IPv6 network with multiple equal-cost paths to the same destination via different interfaces. The router receives traffic from a source that is reachable via one of the paths, but the traffic arrives on a different interface. The traffic 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 source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet.
Correct. Link-local addresses are not in the routing table, so uRPF loose mode drops them.
B
The router has 'ipv6 uRPF strict' configured instead of loose, but the show command indicates loose.
Why wrong: Incorrect. The configuration is loose mode.
C
The equal-cost paths cause the routing table to have multiple entries, and uRPF loose mode requires a single best path.
Why wrong: Incorrect. uRPF loose mode works with multiple paths; it just needs any entry.
D
The interface has an IPv6 ACL that denies the traffic before uRPF is applied.
Why wrong: Incorrect. ACLs are applied before uRPF, but the question implies uRPF is the cause.

Full breakdown with real-world context →

Quick Answer

The answer is that uRPF loose mode drops the traffic because the source address is a link-local address (fe80::), which is not installed in the global routing table. Loose mode only verifies that a matching route exists for the source in the routing table, without caring which interface it arrived on, so if the source is reachable via an equal-cost path on another interface, the check should normally pass. However, link-local addresses are never placed in the global IPv6 routing table—they are only used for neighbor discovery and local link operations—so uRPF loose mode finds no entry and drops the packet, even though the source is technically reachable via a different path. This scenario tests your understanding of how uRPF interacts with IPv6 addressing on the CCNP ENARSI 300-410 exam, where the common trap is assuming that a reachable source automatically has a routing table entry. Remember the tip: “Link-local, no table—loose mode drops the cable.”

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 source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet.

In uRPF loose mode, the router checks that the source address of an incoming packet has a matching entry in the routing table, but it does not require that the packet arrives on the interface used to reach that source. However, link-local addresses (fe80::/10) are not installed in the global IPv6 routing table; they are only reachable on the local link. Therefore, when a packet with a link-local source address arrives, uRPF loose mode finds no matching route and drops the packet, regardless of the interface.

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 source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet.
Why this is correct
Correct. Link-local addresses are not in the routing table, so uRPF loose mode drops them.
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 uRPF strict' configured instead of loose, but the show command indicates loose.
Why it's wrong here
Incorrect. The configuration is loose mode.
✗
The equal-cost paths cause the routing table to have multiple entries, and uRPF loose mode requires a single best path.
Why it's wrong here
Incorrect. uRPF loose mode works with multiple paths; it just needs any entry.
✗
The interface has an IPv6 ACL that denies the traffic before uRPF is applied.
Why it's wrong here
Incorrect. ACLs are applied before uRPF, but the question implies uRPF is the cause.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between uRPF strict and loose modes, but the trap here is that candidates forget link-local addresses are not in the global routing table, leading them to incorrectly attribute the drop to a mode mismatch or routing table issue.

Detailed technical explanation

How to think about this question

uRPF loose mode (RFC 3704) only verifies that the source address is reachable via any interface in the routing table, making it suitable for asymmetric routing scenarios. However, link-local addresses (fe80::/10) are scoped to a single link and never appear in the global routing table, so uRPF always fails for such sources. In IPv6, link-local addresses are mandatory for Neighbor Discovery and other link-local operations, but they cannot be used as source addresses for traffic that must be validated by uRPF across different interfaces.

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.

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 source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet. — In uRPF loose mode, the router checks that the source address of an incoming packet has a matching entry in the routing table, but it does not require that the packet arrives on the interface used to reach that source. However, link-local addresses (fe80::/10) are not installed in the global IPv6 routing table; they are only reachable on the local link. Therefore, when a packet with a link-local source address arrives, uRPF loose mode finds no matching route and drops the packet, regardless of the interface.

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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