Question 1,975 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. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 administrator configures 'ipv6 nd raguard' on a switch port connected to a router. The router is sending Router Advertisements with a non-zero Router Lifetime. The switch logs indicate that RAs are being dropped, and the port goes into err-disable state. The engineer checks the RA Guard policy and sees that the default policy is applied. What is the most likely reason for the drops?

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 RA has a hop-limit less than 255, which RA Guard treats as invalid and drops.
RA Guard expects hop-limit of 255 for locally generated RAs.
B
The RA Guard policy is configured to block all RAs regardless of source.
Why wrong: Default policy blocks RAs from untrusted ports, but not all.
C
The router is using a multicast MAC address that is not allowed by RA Guard.
Why wrong: RA Guard does not filter based on MAC.
D
The switch port is in access mode, and RA Guard only works on trunk ports.
Why wrong: RA Guard works on any port.

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 RA has a hop-limit less than 255, which RA Guard treats as invalid and drops.

The default RA Guard policy on Cisco switches blocks Router Advertisements (RAs) that do not have a hop limit of 255. This is because legitimate routers always send RAs with a hop limit of 255, as specified in RFC 4861. When the router sends an RA with a hop limit less than 255, RA Guard treats it as invalid and drops it, which can also trigger err-disable state on the port.

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 RA has a hop-limit less than 255, which RA Guard treats as invalid and drops.
Why this is correct
RA Guard expects hop-limit of 255 for locally generated RAs.
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 RA Guard policy is configured to block all RAs regardless of source.
Why it's wrong here
Default policy blocks RAs from untrusted ports, but not all.
✗
The router is using a multicast MAC address that is not allowed by RA Guard.
Why it's wrong here
RA Guard does not filter based on MAC.
✗
The switch port is in access mode, and RA Guard only works on trunk ports.
Why it's wrong here
RA Guard works on any port.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the specific hop-limit validation in the default RA Guard policy, where candidates mistakenly think the issue is with port mode or MAC addressing rather than the hop-limit field.

Detailed technical explanation

How to think about this question

RA Guard (Router Advertisement Guard) enforces IPv6 first-hop security by validating RAs against a policy. The default policy requires the hop limit to be exactly 255, as any value less than 255 indicates the RA may have been forwarded by an intermediate device (e.g., a rogue host). If the RA fails validation, the switch can either drop the packet or, if configured with the 'errdisable' action, place the port in err-disable state to prevent further attacks.

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 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.

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 RA has a hop-limit less than 255, which RA Guard treats as invalid and drops. — The default RA Guard policy on Cisco switches blocks Router Advertisements (RAs) that do not have a hop limit of 255. This is because legitimate routers always send RAs with a hop limit of 255, as specified in RFC 4861. When the router sends an RA with a hop limit less than 255, RA Guard treats it as invalid and drops it, which can also trigger err-disable state on the port.

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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