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IPv6 First Hop Security →hardMultiple ChoiceObjective-mapped

300-410 IPv6 First Hop Security Practice Question

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 IPv6 neighbor discovery issues on a VLAN. Router R1 is configured with IPv6 First Hop Security features. Hosts are unable to communicate with each other, even though they have valid IPv6 addresses. Router R1 has the following relevant configuration:

interface Vlan100

ipv6 address 2001:DB8:1:100::1/64 ipv6 nd raguard ipv6 dhcp guard ipv6 source guard !

Router R2 shows: debug ipv6 nd output indicates that Neighbor Solicitations from hosts are being dropped. What is the root 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.

A
RA Guard is blocking Neighbor Solicitations because they are mistaken for RAs.
Why wrong: RA Guard only filters Router Advertisements, not Neighbor Solicitations.
B
DHCP Guard is dropping Neighbor Solicitations because they contain DHCP options.
Why wrong: DHCP Guard filters DHCP messages, not Neighbor Solicitations.
C
IPv6 Source Guard is dropping Neighbor Solicitations because the source address is not in the binding table.
Source Guard validates source addresses against the binding table; if the host is not bound, the NS is dropped.
D
The VLAN interface is not in a state to forward ND messages due to a spanning tree issue.
Why wrong: No spanning tree issue is indicated; the debug shows drops due to security features.

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

✓

IPv6 Source Guard is dropping Neighbor Solicitations because the source address is not in the binding table.

The correct answer is C because IPv6 Source Guard (ipv6 source guard) on the VLAN interface drops any IPv6 packet whose source address is not present in the IPv6 neighbor binding table. When hosts send Neighbor Solicitations, their source IPv6 addresses are not yet in the binding table (since the NS is the first step in address resolution), causing the packets to be dropped. This prevents hosts from completing the neighbor discovery process, even though they have valid IPv6 addresses.

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.

✗
RA Guard is blocking Neighbor Solicitations because they are mistaken for RAs.
Why it's wrong here
RA Guard only filters Router Advertisements, not Neighbor Solicitations.
✗
DHCP Guard is dropping Neighbor Solicitations because they contain DHCP options.
Why it's wrong here
DHCP Guard filters DHCP messages, not Neighbor Solicitations.
✓
IPv6 Source Guard is dropping Neighbor Solicitations because the source address is not in the binding table.
Why this is correct
Source Guard validates source addresses against the binding table; if the host is not bound, the NS is dropped.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The VLAN interface is not in a state to forward ND messages due to a spanning tree issue.
Why it's wrong here
No spanning tree issue is indicated; the debug shows drops due to security features.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that RA Guard or DHCP Guard drops Neighbor Solicitations, but the trap here is that IPv6 Source Guard is the only FHS feature that validates the source address of all IPv6 packets, including ND messages, and it drops them if no binding exists.

Trap categories for this question

Command / output trap
No spanning tree issue is indicated; the debug shows drops due to security features.

Detailed technical explanation

How to think about this question

IPv6 Source Guard works by maintaining a binding table built from DHCPv6 snooping or Neighbor Discovery snooping; when a host sends a Neighbor Solicitation, the source address must match a valid binding entry, otherwise the packet is dropped. In this scenario, the NS is the first packet from the host, so no binding exists yet, causing the drop. A real-world workaround is to enable 'ipv6 nd prefix' or adjust the source guard policy to allow NS messages from unknown sources, but the default behavior is strict.

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.

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 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: IPv6 Source Guard is dropping Neighbor Solicitations because the source address is not in the binding table. — The correct answer is C because IPv6 Source Guard (ipv6 source guard) on the VLAN interface drops any IPv6 packet whose source address is not present in the IPv6 neighbor binding table. When hosts send Neighbor Solicitations, their source IPv6 addresses are not yet in the binding table (since the NS is the first step in address resolution), causing the packets to be dropped. This prevents hosts from completing the neighbor discovery process, even though they have valid IPv6 addresses.

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". 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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