Question 51 of 2,152

IPv6 First Hop Security →mediumMultiple 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 an issue where IPv6 traffic is being forwarded incorrectly on a switch. The switch is configured with IPv6 Source Guard on access ports. A legitimate host on port Fa0/1 with IPv6 address 2001:db8:1::10 is unable to send traffic to the default gateway. The engineer checks the IPv6 binding table and sees that the host's entry is missing. 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.

A
The host is using a static IPv6 address, and ND snooping is not enabled on the VLAN, so the binding was never learned.
Correct because IPv6 Source Guard relies on ND snooping to learn static addresses; without it, the host's traffic is dropped.
B
The host's MAC address is not in the MAC address table for VLAN 1.
Why wrong: Incorrect because the MAC address table is for Layer 2 forwarding; IPv6 Source Guard operates at Layer 3 and uses the binding table.
C
The switch is running IPv6 First Hop Security in monitor mode, which logs violations but does not drop traffic.
Why wrong: Incorrect because in monitor mode, traffic is not dropped; the symptom is that traffic is not being forwarded.
D
The default gateway router is not sending Router Advertisements, so the host cannot form a default route.
Why wrong: Incorrect because the issue is about the host's traffic being dropped by the switch, not about the host's routing table.

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 host is using a static IPv6 address, and ND snooping is not enabled on the VLAN, so the binding was never learned.

IPv6 Source Guard relies on the IPv6 binding table, which is populated by IPv6 Neighbor Discovery (ND) snooping. If the host uses a static IPv6 address and ND snooping is not enabled on the VLAN, the switch never learns the binding, so IPv6 Source Guard drops the traffic as unauthorized. Enabling ND snooping allows the switch to inspect Neighbor Solicitation and Advertisement messages to build the binding table dynamically.

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 host is using a static IPv6 address, and ND snooping is not enabled on the VLAN, so the binding was never learned.
Why this is correct
Correct because IPv6 Source Guard relies on ND snooping to learn static addresses; without it, the host's traffic is dropped.
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 host's MAC address is not in the MAC address table for VLAN 1.
Why it's wrong here
Incorrect because the MAC address table is for Layer 2 forwarding; IPv6 Source Guard operates at Layer 3 and uses the binding table.
✗
The switch is running IPv6 First Hop Security in monitor mode, which logs violations but does not drop traffic.
Why it's wrong here
Incorrect because in monitor mode, traffic is not dropped; the symptom is that traffic is not being forwarded.
✗
The default gateway router is not sending Router Advertisements, so the host cannot form a default route.
Why it's wrong here
Incorrect because the issue is about the host's traffic being dropped by the switch, not about the host's routing table.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the dependency of IPv6 Source Guard on ND snooping, and the trap here is that candidates assume IPv6 Source Guard works independently or that static addresses are automatically learned, when in fact the binding table must be populated either dynamically via ND snooping or manually.

Detailed technical explanation

How to think about this question

IPv6 Source Guard uses the binding table built by ND snooping to validate source IPv6 addresses in data packets; if ND snooping is disabled, the switch cannot learn static addresses unless they are manually configured via the `ipv6 source binding` command. In a real-world scenario, a network engineer might forget to enable `ipv6 nd snooping` on the VLAN after deploying static IPv6 hosts, causing silent traffic drops that are difficult to trace without checking the binding table. The binding table is also used by other FHS features like IPv6 RA Guard and DHCPv6 Guard, making ND snooping a prerequisite for multiple security mechanisms.

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: The host is using a static IPv6 address, and ND snooping is not enabled on the VLAN, so the binding was never learned. — IPv6 Source Guard relies on the IPv6 binding table, which is populated by IPv6 Neighbor Discovery (ND) snooping. If the host uses a static IPv6 address and ND snooping is not enabled on the VLAN, the switch never learns the binding, so IPv6 Source Guard drops the traffic as unauthorized. Enabling ND snooping allows the switch to inspect Neighbor Solicitation and Advertisement messages to build the binding table dynamically.

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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