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.
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.
RA Guard by default uses a policy that blocks RAs from all ports except those explicitly configured as 'trusted'. Even if the router is legitimate, the port must be trusted. However, the edge case here is that the default RA Guard policy also checks the 'hop-limit' field in the RA. If the router sends RAs with a hop-limit other than 255 (the default for locally generated packets), RA Guard will drop them. This can happen if the router is multiple hops away or if the RA is forwarded (e.g., via a tunnel). The most common misconfiguration is that the router's RA has a hop-limit less than 255, which is considered invalid by RA Guard.
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 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
Access ports place end devices into a single VLAN.
✗
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: 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
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 RA has a hop-limit less than 255, which RA Guard treats as invalid and drops. — RA Guard by default uses a policy that blocks RAs from all ports except those explicitly configured as 'trusted'. Even if the router is legitimate, the port must be trusted. However, the edge case here is that the default RA Guard policy also checks the 'hop-limit' field in the RA. If the router sends RAs with a hop-limit other than 255 (the default for locally generated packets), RA Guard will drop them. This can happen if the router is multiple hops away or if the RA is forwarded (e.g., via a tunnel). The most common misconfiguration is that the router's RA has a hop-limit less than 255, which is considered invalid by RA Guard.
What should I do if I get this 300-410 question wrong?
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.
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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