- A
DHCPv6 guard is allowing client messages but blocking server messages from untrusted sources, preventing rogue DHCPv6 servers.
The ADVERTISE from fe80::4 is blocked, which is typical for DHCPv6 guard on untrusted ports.
- B
DHCPv6 guard is blocking all DHCPv6 messages, indicating a misconfiguration.
Why wrong: The SOLICIT is allowed, so not all messages are blocked.
- C
DHCPv6 guard is allowing all messages but logging them for analysis.
Why wrong: The ADVERTISE is explicitly blocked, so not all are allowed.
- D
DHCPv6 guard is not configured; the debug output is from default DHCPv6 behavior.
Why wrong: The debug references policy DHCP-POLICY, indicating DHCPv6 guard is configured.
Question 564 of 2,152
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 runs the following command to troubleshoot DHCPv6 guard:
R1# debug ipv6 dhcp guard *Mar 1 00:03:45.678: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 SOLICIT from fe80::3, client DUID 00010001abcd1234 *Mar 1 00:03:45.678: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 SOLICIT from fe80::3 is allowed by policy DHCP-POLICY *Mar 1 00:03:46.901: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 ADVERTISE from fe80::4, server DUID 0001000156789012 *Mar 1 00:03:46.901: IPv6-DHCP-Guard: R1, Fa0/0, DHCPv6 ADVERTISE from fe80::4 is blocked by policy DHCP-POLICY
What does this output indicate?
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
✓
DHCPv6 guard is allowing client messages but blocking server messages from untrusted sources, preventing rogue DHCPv6 servers.
The debug output shows that DHCPv6 SOLICIT messages from client fe80::3 are allowed by policy DHCP-POLICY, while DHCPv6 ADVERTISE messages from server fe80::4 are blocked by the same policy. This is the expected behavior of DHCPv6 guard: it permits client messages (SOLICIT, REQUEST, etc.) to reach potential servers, but it blocks server messages (ADVERTISE, REPLY, etc.) from untrusted ports to prevent rogue DHCPv6 servers from assigning malicious configurations. Option A correctly identifies this selective filtering.
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.
- ✓
DHCPv6 guard is allowing client messages but blocking server messages from untrusted sources, preventing rogue DHCPv6 servers.
Why this is correct
The ADVERTISE from fe80::4 is blocked, which is typical for DHCPv6 guard on untrusted ports.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
DHCPv6 guard is blocking all DHCPv6 messages, indicating a misconfiguration.
Why it's wrong here
The SOLICIT is allowed, so not all messages are blocked.
- ✗
DHCPv6 guard is allowing all messages but logging them for analysis.
Why it's wrong here
The ADVERTISE is explicitly blocked, so not all are allowed.
- ✗
DHCPv6 guard is not configured; the debug output is from default DHCPv6 behavior.
Why it's wrong here
The debug references policy DHCP-POLICY, indicating DHCPv6 guard is configured.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that DHCPv6 guard blocks all DHCPv6 traffic, when in fact it only blocks server messages from untrusted sources, allowing client messages to pass through.
Detailed technical explanation
How to think about this question
DHCPv6 guard is an IPv6 First Hop Security feature that inspects DHCPv6 server messages (ADVERTISE, REPLY, RECONFIGURE) on a per-interface basis, using a policy to define trusted and untrusted ports. The policy can be configured with the `ipv6 dhcp guard policy` command, where you can specify `trusted` or `untrusted` ports; by default, all ports are untrusted, meaning server messages are dropped unless explicitly trusted. In this scenario, the policy DHCP-POLICY likely has the interface Fa0/0 configured as untrusted for server messages, while client messages are always allowed to ensure legitimate clients can still send SOLICIT messages to reach a trusted server elsewhere.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
Visual reference
What to study next
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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: DHCPv6 guard is allowing client messages but blocking server messages from untrusted sources, preventing rogue DHCPv6 servers. — The debug output shows that DHCPv6 SOLICIT messages from client fe80::3 are allowed by policy DHCP-POLICY, while DHCPv6 ADVERTISE messages from server fe80::4 are blocked by the same policy. This is the expected behavior of DHCPv6 guard: it permits client messages (SOLICIT, REQUEST, etc.) to reach potential servers, but it blocks server messages (ADVERTISE, REPLY, etc.) from untrusted ports to prevent rogue DHCPv6 servers from assigning malicious configurations. Option A correctly identifies this selective filtering.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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