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 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.
The 'ipv6 nd router-preference high' command sets the preference in RAs, but if the host's operating system or implementation does not support RFC 4191 (Default Router Preferences), it may ignore the preference field. However, the more common issue is that R1's RAs are being suppressed or filtered. The correct answer identifies that R1 has an IPv6 ACL blocking outgoing RAs, a subtle interaction between First Hop Security and ACLs.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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
Standard ACLs match source addresses.
✗
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: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
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
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
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 ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
IPv6 First Hop Security — This question tests IPv6 First Hop Security — Standard ACLs match source addresses..
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. — The 'ipv6 nd router-preference high' command sets the preference in RAs, but if the host's operating system or implementation does not support RFC 4191 (Default Router Preferences), it may ignore the preference field. However, the more common issue is that R1's RAs are being suppressed or filtered. The correct answer identifies that R1 has an IPv6 ACL blocking outgoing RAs, a subtle interaction between First Hop Security and ACLs.
What should I do if I get this 300-410 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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