- A
Router R1's RA interval is set too high, causing hosts to prefer R2's more frequent RAs.
Why wrong: The RA interval is not shown; preference should override frequency per RFC.
- B
An IPv6 ACL applied to the interface is blocking Router Advertisements from R1.
An ACL with an implicit deny can block RAs, even if the router is configured to send them with high preference.
- C
Router R2 is configured with 'ipv6 nd router-preference high' as well, overriding R1's preference.
Why wrong: The debug shows R2 sends medium preference, not high.
- D
Hosts are configured to ignore router preference due to a security policy.
Why wrong: Hosts typically follow RFC 4191 unless explicitly configured otherwise.
Question 1,705 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 is troubleshooting IPv6 connectivity issues on a multi-access segment where Router R1 and Router R2 are both acting as default routers. Hosts on the segment are not using R1 as a preferred router, even though R1 has a higher router preference. Router R1 has the following relevant configuration:
interface GigabitEthernet0/0
ipv6 address 2001:DB8:1::1/64 ipv6 nd router-preference high !
Router R2 shows: debug ipv6 nd output indicates that R2 is sending RAs with default preference (medium). What is the root cause?
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 the interface is blocking Router Advertisements from R1.
Option B is correct because an IPv6 ACL applied to the interface can filter Router Advertisement (RA) messages. If R1's RAs are blocked, hosts will never receive the high-preference indication and will instead use R2's default-preference RAs, causing R1 not to be preferred. The debug output confirms R2 is sending RAs with medium preference, and R1's configuration explicitly sets preference high, so the only explanation for hosts ignoring R1's preference is that its RAs are not reaching them.
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.
- ✗
Router R1's RA interval is set too high, causing hosts to prefer R2's more frequent RAs.
Why it's wrong here
The RA interval is not shown; preference should override frequency per RFC.
- ✓
An IPv6 ACL applied to the interface is blocking Router Advertisements from R1.
Why this is correct
An ACL with an implicit deny can block RAs, even if the router is configured to send them with high preference.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Router R2 is configured with 'ipv6 nd router-preference high' as well, overriding R1's preference.
Why it's wrong here
The debug shows R2 sends medium preference, not high.
- ✗
Hosts are configured to ignore router preference due to a security policy.
Why it's wrong here
Hosts typically follow RFC 4191 unless explicitly configured otherwise.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that router preference alone guarantees selection, but the trap here is that an ACL or other filter can silently drop RAs, making the preference irrelevant because hosts never receive the RA.
Trap categories for this question
Command / output trap
The RA interval is not shown; preference should override frequency per RFC.
Detailed technical explanation
How to think about this question
Router Advertisement messages carry a 2-bit router preference field (RFC 4191) that can be high (01), medium (00), or low (11). Hosts use this field to select a default router when multiple routers are present. An IPv6 ACL applied to the interface (e.g., with 'ipv6 traffic-filter' or 'access-list' under the interface) can filter ICMPv6 type 134 (RA) packets, preventing hosts from learning the preference. In real-world scenarios, misapplied ACLs are a common cause of asymmetric routing or suboptimal default gateway selection.
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: An IPv6 ACL applied to the interface is blocking Router Advertisements from R1. — Option B is correct because an IPv6 ACL applied to the interface can filter Router Advertisement (RA) messages. If R1's RAs are blocked, hosts will never receive the high-preference indication and will instead use R2's default-preference RAs, causing R1 not to be preferred. The debug output confirms R2 is sending RAs with medium preference, and R1's configuration explicitly sets preference high, so the only explanation for hosts ignoring R1's preference is that its RAs are not reaching them.
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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