- A
The ACL denies ICMPv6 types 133-137, which are required for Neighbor Discovery, causing the router to fail to resolve neighbors.
Correct. ND uses ICMPv6 types 133-137; denying them breaks IPv6 connectivity.
- B
The ACL must be applied outbound, not inbound, to allow ND packets to be sent.
Why wrong: Incorrect. ND packets are both sent and received; inbound ACL filters incoming packets, which include ND solicitations and advertisements.
- C
The router needs to have 'ipv6 nd suppress' configured to bypass ACL filtering for ND packets.
Why wrong: Incorrect. There is no such command; ND packets are subject to ACL filtering unless explicitly permitted.
- D
The ACL should use 'permit ipv6 any any' before the deny statements to allow ND, but the engineer placed it after.
Why wrong: Incorrect. Even if 'permit ipv6 any any' is placed, it would permit all traffic, including ND, but the question states the ACL permits only ICMPv6 echo types, so ND is still denied.
Quick Answer
The answer is that the ACL denies ICMPv6 types 133 through 137, which are the core messages required for IPv6 Neighbor Discovery (ND). Without these specific ICMPv6 types—router solicitation (133), router advertisement (134), neighbor solicitation (135), neighbor advertisement (136), and redirect (137)—the router cannot resolve the link-layer addresses of neighboring devices or maintain IPv6 connectivity. This is a classic trap on the Cisco CCNP ENARSI 300-410 exam, testing your understanding that IPv6 ACLs must explicitly permit ND traffic alongside any other permitted ICMP types like echo request and echo reply. Many engineers mistakenly block all ICMPv6 except ping, forgetting that ND is not optional but essential for IPv6 operation. On the exam, remember the memory tip: “ND needs 133 to 137—without them, your neighbor is in heaven.”
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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.
An engineer configures an IPv6 ACL on a router interface to permit only specific ICMPv6 types (e.g., echo request and echo reply) and deny all other IPv6 traffic. After applying the ACL inbound, the router stops forming IPv6 neighbor discoveries (ND) and the interface loses IPv6 connectivity. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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
The ACL denies ICMPv6 types 133-137, which are required for Neighbor Discovery, causing the router to fail to resolve neighbors.
IPv6 Neighbor Discovery (ND) uses ICMPv6 types 133-137 (router solicitation, router advertisement, neighbor solicitation, neighbor advertisement, redirect). If the ACL denies all ICMPv6 types except echo, ND packets are dropped, preventing address resolution and neighbor reachability. This is a common misconfiguration: engineers forget that ND is essential for IPv6 operation and must be permitted.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 ACL denies ICMPv6 types 133-137, which are required for Neighbor Discovery, causing the router to fail to resolve neighbors.
Why this is correct
Correct. ND uses ICMPv6 types 133-137; denying them breaks IPv6 connectivity.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The ACL must be applied outbound, not inbound, to allow ND packets to be sent.
Why it's wrong here
Incorrect. ND packets are both sent and received; inbound ACL filters incoming packets, which include ND solicitations and advertisements.
- ✗
The router needs to have 'ipv6 nd suppress' configured to bypass ACL filtering for ND packets.
Why it's wrong here
Incorrect. There is no such command; ND packets are subject to ACL filtering unless explicitly permitted.
- ✗
The ACL should use 'permit ipv6 any any' before the deny statements to allow ND, but the engineer placed it after.
Why it's wrong here
Incorrect. Even if 'permit ipv6 any any' is placed, it would permit all traffic, including ND, but the question states the ACL permits only ICMPv6 echo types, so ND is still denied.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
Incorrect. There is no such command; ND packets are subject to ACL filtering unless explicitly permitted.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
- →
IPv6 Traffic Filtering and uRPF — study guide chapter
Learn the concepts, then practise the questions
- →
IPv6 Traffic Filtering and uRPF practice questions
Targeted practice on this topic area only
- →
All 300-410 questions
2,152 questions across all exam domains
- →
Cisco CCNP ENARSI 300-410 study guide
Full concept coverage aligned to exam objectives
- →
300-410 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related 300-410 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Layer 3 Technologies practice questions
Practise 300-410 questions linked to Layer 3 Technologies.
EIGRP Troubleshooting practice questions
Practise 300-410 questions linked to EIGRP Troubleshooting.
OSPF Troubleshooting (v2/v3) practice questions
Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).
BGP Troubleshooting practice questions
Practise 300-410 questions linked to BGP Troubleshooting.
Route Redistribution practice questions
Practise 300-410 questions linked to Route Redistribution.
Policy-Based Routing (PBR) practice questions
Practise 300-410 questions linked to Policy-Based Routing (PBR).
VRF-Lite practice questions
Practise 300-410 questions linked to VRF-Lite.
Route Maps and Route Filtering practice questions
Practise 300-410 questions linked to Route Maps and Route Filtering.
Administrative Distance practice questions
Practise 300-410 questions linked to Administrative Distance.
Route Summarization practice questions
Practise 300-410 questions linked to Route Summarization.
Bidirectional Forwarding Detection (BFD) practice questions
Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).
VPN Technologies practice questions
Practise 300-410 questions linked to VPN Technologies.
Practice this exam
Start a free 300-410 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The ACL denies ICMPv6 types 133-137, which are required for Neighbor Discovery, causing the router to fail to resolve neighbors. — IPv6 Neighbor Discovery (ND) uses ICMPv6 types 133-137 (router solicitation, router advertisement, neighbor solicitation, neighbor advertisement, redirect). If the ACL denies all ICMPv6 types except echo, ND packets are dropped, preventing address resolution and neighbor reachability. This is a common misconfiguration: engineers forget that ND is essential for IPv6 operation and must be permitted.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.