- A
The 'ipv6 nd raguard' command on the interface filters DHCPv6 SOLICIT messages, preventing relay.
Why wrong: RA Guard filters Router Advertisements, not DHCPv6 messages.
- B
The DHCPv6 relay destination is in a different VRF, and the relay is not configured to use that VRF.
Why wrong: No VRF configuration is shown; the relay destination is reachable via the global table.
- C
An IPv6 ACL applied to GigabitEthernet0/0.100 has an implicit deny that blocks the relayed DHCPv6 traffic.
The implicit deny at the end of an IPv6 ACL can block DHCPv6 relay packets if no explicit permit statement exists for the relay destination.
- D
The DHCPv6 server is not configured to accept relayed messages from this relay agent.
Why wrong: DHCPv6 servers accept relayed messages by default; the issue is on the relay side.
Quick Answer
The answer is an implicit deny in an IPv6 ACL applied to the relay interface. This is correct because, while the debug output confirms DHCPv6 SOLICIT messages are being relayed from VLAN 100, they never reach the server, indicating the relayed traffic is being silently dropped by an ACL’s implicit deny-all rule rather than a routing or configuration issue. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPv6 ACLs interact with DHCPv6 relay and First Hop Security features like RA Guard—a common trap is assuming RA Guard blocks DHCP traffic, when in fact it only filters Router Advertisements. The key insight is that any IPv6 ACL, even one applied for security, ends with an implicit deny that will block relayed DHCPv6 packets unless an explicit permit statement is added. Memory tip: “RA Guard guards RAs, not relays—check your ACL permits for DHCPv6 relays.”
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 large enterprise network is experiencing intermittent IPv6 connectivity loss for hosts on VLAN 100. Router R1 has the following relevant configuration:
interface GigabitEthernet0/0.100 encapsulation dot1Q 100
ipv6 address 2001:DB8:1:100::1/64 ipv6 nd raguard ipv6 nd prefix default ipv6 dhcp relay destination 2001:DB8:1:200::1 !
Router R2 shows: debug ipv6 dhcp relay output indicates that DHCPv6 requests from VLAN 100 are being relayed, but the server never receives the SOLICIT messages. What is the root cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"never"Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
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
An IPv6 ACL applied to GigabitEthernet0/0.100 has an implicit deny that blocks the relayed DHCPv6 traffic.
The issue is that the DHCPv6 relay agent is configured with 'ipv6 nd raguard' which filters Router Advertisement messages but does not affect DHCPv6 relay. However, the relay destination is unreachable due to a missing route or ACL. The correct answer identifies that an implicit deny in an IPv6 ACL applied to the relay interface is blocking the relayed traffic, a common oversight when combining First Hop Security features with ACLs.
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 'ipv6 nd raguard' command on the interface filters DHCPv6 SOLICIT messages, preventing relay.
Why it's wrong here
RA Guard filters Router Advertisements, not DHCPv6 messages.
- ✗
The DHCPv6 relay destination is in a different VRF, and the relay is not configured to use that VRF.
Why it's wrong here
No VRF configuration is shown; the relay destination is reachable via the global table.
- ✓
An IPv6 ACL applied to GigabitEthernet0/0.100 has an implicit deny that blocks the relayed DHCPv6 traffic.
Why this is correct
The implicit deny at the end of an IPv6 ACL can block DHCPv6 relay packets if no explicit permit statement exists for the relay destination.
Clue confirmation
The clue word "never" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The DHCPv6 server is not configured to accept relayed messages from this relay agent.
Why it's wrong here
DHCPv6 servers accept relayed messages by default; the issue is on the relay side.
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.
Trap categories for this question
Command / output trap
No VRF configuration is shown; the relay destination is reachable via the global table.
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: An IPv6 ACL applied to GigabitEthernet0/0.100 has an implicit deny that blocks the relayed DHCPv6 traffic. — The issue is that the DHCPv6 relay agent is configured with 'ipv6 nd raguard' which filters Router Advertisement messages but does not affect DHCPv6 relay. However, the relay destination is unreachable due to a missing route or ACL. The correct answer identifies that an implicit deny in an IPv6 ACL applied to the relay interface is blocking the relayed traffic, a common oversight when combining First Hop Security features with ACLs.
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: "never". Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 18, 2026
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
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