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.

Question 1mediummultiple choice

Study the full IPv6 explanation →

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 requires a valid binding entry (learned via DHCPv6 snooping or ND snooping) to permit traffic. If the host is using a static IPv6 address, ND snooping must be enabled to learn the binding; otherwise, traffic is dropped.

Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.

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
Access ports place end devices into a single VLAN.
✗
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: an active trunk can still block the VLAN you need

A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.

Detailed technical explanation

How to think about this question

VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.

KKey Concepts to Remember

Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.

TExam Day Tips

Use show vlan brief to verify access VLANs.
Use show interfaces trunk to verify trunk state and allowed VLANs.
Do not treat every same-VLAN issue as a routing problem.

Key takeaway

A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.

Real-world example

How this comes up in practice

A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.

What to study next

Got this wrong? Here's your next step.

Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.

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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 — Access ports place end devices into a single VLAN..

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 requires a valid binding entry (learned via DHCPv6 snooping or ND snooping) to permit traffic. If the host is using a static IPv6 address, ND snooping must be enabled to learn the binding; otherwise, traffic is dropped.

What should I do if I get this 300-410 question wrong?

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?

Access ports place end devices into a single VLAN.

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