Question 92 of 2,152

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 enables IPv6 First Hop Security with 'ipv6 dhcp guard' on a switch port connected to a legitimate DHCPv6 server. Clients on other ports receive DHCPv6 replies, but the server's port is being err-disabled repeatedly. The engineer checks the logs and sees DHCPv6 server advertisements being dropped. 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
The port is not configured as 'trusted' for DHCPv6 Guard, causing all server advertisements to be dropped.
DHCPv6 Guard requires explicit trust for server ports.
B
The DHCPv6 server is sending messages with an invalid DUID.
Why wrong: Invalid DUID would not cause err-disable; it would just be ignored.
C
DHCPv6 Guard only works with stateful DHCPv6, not stateless.
Why wrong: DHCPv6 Guard works with both stateful and stateless.
D
The switch is running an older IOS version that does not support DHCPv6 Guard.
Why wrong: If unsupported, the command would be rejected.

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 port is not configured as 'trusted' for DHCPv6 Guard, causing all server advertisements to be dropped.

When 'ipv6 dhcp guard' is enabled on a switch port, all DHCPv6 server advertisements (REPLY and ADVERTISE messages) are dropped by default unless the port is explicitly configured as 'trusted'. Since the server's port is untrusted, the switch drops the legitimate server's advertisements, causing the port to be err-disabled due to repeated violations. Configuring 'ipv6 dhcp guard trust' on the server-facing port resolves this issue.

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 port is not configured as 'trusted' for DHCPv6 Guard, causing all server advertisements to be dropped.
Why this is correct
DHCPv6 Guard requires explicit trust for server ports.
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.
✗
The DHCPv6 server is sending messages with an invalid DUID.
Why it's wrong here
Invalid DUID would not cause err-disable; it would just be ignored.
✗
DHCPv6 Guard only works with stateful DHCPv6, not stateless.
Why it's wrong here
DHCPv6 Guard works with both stateful and stateless.
✗
The switch is running an older IOS version that does not support DHCPv6 Guard.
Why it's wrong here
If unsupported, the command would be rejected.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the default untrusted behavior of DHCPv6 Guard, where candidates mistakenly assume that enabling the feature on a port automatically allows server traffic, rather than requiring an explicit 'trust' keyword.

Trap categories for this question

Command / output trap
If unsupported, the command would be rejected.

Detailed technical explanation

How to think about this question

DHCPv6 Guard is part of the IPv6 First Hop Security suite and uses a trust model: by default, all ports are untrusted, and any DHCPv6 server message received on an untrusted port is dropped. The err-disabled state occurs because the switch counts the dropped packets as a security violation (similar to port security). In real-world deployments, forgetting to set the server port as trusted is a common misconfiguration, especially when enabling multiple FHS features like RA Guard or ND Inspection simultaneously.

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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

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 port is not configured as 'trusted' for DHCPv6 Guard, causing all server advertisements to be dropped. — When 'ipv6 dhcp guard' is enabled on a switch port, all DHCPv6 server advertisements (REPLY and ADVERTISE messages) are dropped by default unless the port is explicitly configured as 'trusted'. Since the server's port is untrusted, the switch drops the legitimate server's advertisements, causing the port to be err-disabled due to repeated violations. Configuring 'ipv6 dhcp guard trust' on the server-facing port resolves this issue.

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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