Question 673 of 2,152

IPv6 First Hop Security →hardMultiple ChoiceObjective-mapped

DHCPv6 Relay Failure — Implicit Deny IPv6 ACL | Cisco CCNP ENARSI 300-410

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 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.

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.

Full breakdown with real-world context →

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.”

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.

Option C is correct because an IPv6 ACL applied to the interface can implicitly deny the relayed DHCPv6 traffic. The debug output shows that DHCPv6 requests are being relayed from R1, but the server never receives the SOLICIT messages, indicating that the relayed packets are being dropped before leaving the router. An implicit deny in an IPv6 ACL on the outgoing interface (or inbound on the next hop) would block the relayed DHCPv6 unicast traffic, while the local relay process still sees the original client messages.

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 '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
Read the scenario before looking for a memorised answer.
✗
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: answer the scenario, not the keyword

The trap here is that candidates confuse the 'ipv6 nd raguard' command with a DHCPv6 filter, when in reality it only applies to Router Advertisement messages, not DHCPv6 packets.

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

DHCPv6 relay agents forward client SOLICIT messages as unicast RELAY-FORW messages to the server. If an IPv6 ACL is applied on the relay agent's outgoing interface (or inbound on the next-hop interface), it must explicitly permit the relayed DHCPv6 traffic (UDP port 547) to the server's address; otherwise, the implicit deny at the end of the ACL will drop the packets. This is a common misconfiguration because engineers often focus on client-facing ACLs but forget that relayed traffic uses different source/destination addresses and ports.

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.

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: An IPv6 ACL applied to GigabitEthernet0/0.100 has an implicit deny that blocks the relayed DHCPv6 traffic. — Option C is correct because an IPv6 ACL applied to the interface can implicitly deny the relayed DHCPv6 traffic. The debug output shows that DHCPv6 requests are being relayed from R1, but the server never receives the SOLICIT messages, indicating that the relayed packets are being dropped before leaving the router. An implicit deny in an IPv6 ACL on the outgoing interface (or inbound on the next hop) would block the relayed DHCPv6 unicast traffic, while the local relay process still sees the original client messages.

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: "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?

Read the scenario before looking for a memorised answer.

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