Question 935 of 2,152

IPv6 First Hop Security →hardMultiple ChoiceObjective-mapped

IPv6 ND Inspection DAD Failure: Troubleshooting Neighbor Solicitation Drops

This 300-410 practice question tests your understanding of ipv6 first hop security. 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 an issue where IPv6 hosts are unable to perform Duplicate Address Detection (DAD) successfully. The switch is configured with IPv6 First Hop Security features including ND Inspection and ND Suppress. The engineer notices that Neighbor Solicitation messages for DAD are being dropped by the switch. 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: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
ND Inspection is configured to drop Neighbor Solicitations with an unspecified source address (::) because it has no binding for that address.
Correct because ND Inspection typically requires a valid binding for the source address; DAD uses :: as source, which is not in the binding table, causing drops.
B
RA Guard is configured to drop all multicast traffic, including Neighbor Solicitations.
Why wrong: Incorrect because RA Guard only filters Router Advertisements and Redirect messages, not Neighbor Solicitations.
C
DHCPv6 Guard is blocking the DAD messages because they are considered DHCPv6 traffic.
Why wrong: Incorrect because DHCPv6 Guard only filters DHCPv6 messages, not Neighbor Discovery messages.
D
IPv6 Source Guard is dropping the DAD messages because the source address :: is not in the binding table.
Why wrong: Incorrect because IPv6 Source Guard filters data traffic based on source address, but DAD messages are control plane and typically not filtered by Source Guard; ND Inspection is the correct feature.

Full breakdown with real-world context →

Quick Answer

The answer is that ND Inspection is configured to drop Neighbor Solicitations with an unspecified source address (::) because it has no binding for that address. This occurs because Duplicate Address Detection (DAD) inherently uses the unspecified source address (::) when a host probes for a duplicate address, and ND Inspection, which validates traffic against the binding table, sees no entry for that source and drops the packet. In the context of the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of IPv6 First Hop Security interactions, specifically how ND Inspection and ND Suppress work together—a common trap is confusing ND Suppress (which only suppresses advertisements) with ND Inspection (which enforces bindings). The key distinction is that DAD solicitations are dropped not by ND Suppress, but by ND Inspection’s default behavior toward unbound unspecified addresses. Memory tip: DAD uses ::, and ND Inspection says “no binding, no passing.”

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

✓

ND Inspection is configured to drop Neighbor Solicitations with an unspecified source address (::) because it has no binding for that address.

ND Inspection drops Neighbor Solicitations with an unspecified source address (::) because it requires a valid binding for the source address in its binding table. During Duplicate Address Detection (DAD), the source address is :: (RFC 4862), which has no corresponding binding, causing ND Inspection to drop the message and preventing DAD from completing.

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.

✓
ND Inspection is configured to drop Neighbor Solicitations with an unspecified source address (::) because it has no binding for that address.
Why this is correct
Correct because ND Inspection typically requires a valid binding for the source address; DAD uses :: as source, which is not in the binding table, causing drops.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
RA Guard is configured to drop all multicast traffic, including Neighbor Solicitations.
Why it's wrong here
Incorrect because RA Guard only filters Router Advertisements and Redirect messages, not Neighbor Solicitations.
✗
DHCPv6 Guard is blocking the DAD messages because they are considered DHCPv6 traffic.
Why it's wrong here
Incorrect because DHCPv6 Guard only filters DHCPv6 messages, not Neighbor Discovery messages.
✗
IPv6 Source Guard is dropping the DAD messages because the source address :: is not in the binding table.
Why it's wrong here
Incorrect because IPv6 Source Guard filters data traffic based on source address, but DAD messages are control plane and typically not filtered by Source Guard; ND Inspection is the correct feature.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between ND Inspection and IPv6 Source Guard, where candidates mistakenly think Source Guard drops DAD messages, but it is actually ND Inspection that drops Neighbor Solicitations with an unspecified source address due to missing bindings.

Detailed technical explanation

How to think about this question

ND Inspection builds a binding table by snooping Neighbor Discovery messages; DAD Neighbor Solicitations with source :: are intentionally dropped because they lack a binding, a security measure to prevent address spoofing. This behavior is defined in RFC 6621 and Cisco's implementation of IPv6 First Hop Security, where ND Suppress further optimizes by suppressing unnecessary ND messages. In real-world scenarios, this can cause hosts to fail DAD, leading to address assignment failures and connectivity issues.

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 First Hop Security — This question tests IPv6 First Hop Security — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: ND Inspection is configured to drop Neighbor Solicitations with an unspecified source address (::) because it has no binding for that address. — ND Inspection drops Neighbor Solicitations with an unspecified source address (::) because it requires a valid binding for the source address in its binding table. During Duplicate Address Detection (DAD), the source address is :: (RFC 4862), which has no corresponding binding, causing ND Inspection to drop the message and preventing DAD from completing.

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: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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