Question 1,918 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

IPv6 ACL Blocking Neighbor Discovery — ICMPv6 Types

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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.

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.

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.

Full breakdown with real-world context →

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.”

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.

The ACL denies ICMPv6 types 133-137, which are essential for Neighbor Discovery (ND) processes such as Router Solicitation (133), Router Advertisement (134), Neighbor Solicitation (135), Neighbor Advertisement (136), and Redirect (137). Without permitting these types, the router cannot resolve IPv6 neighbors or maintain IPv6 connectivity, as ND is fundamental to IPv6 operation.

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

Cisco often tests the candidate's awareness that IPv6 Neighbor Discovery uses specific ICMPv6 types (133-137) which must be explicitly permitted in ACLs, as they are not automatically allowed like in IPv4 ARP.

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

IPv6 Neighbor Discovery (ND) relies on ICMPv6 types 133-137 as defined in RFC 4861. When an ACL is applied inbound on an interface, it filters all incoming packets before they reach the ND process; if these critical types are denied, the router cannot build or maintain its neighbor cache (similar to ARP in IPv4). In real-world scenarios, this is a common misconfiguration when engineers focus only on echo request/reply and forget that ND is essential for link-local communication and address resolution.

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 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.

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 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..

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. — The ACL denies ICMPv6 types 133-137, which are essential for Neighbor Discovery (ND) processes such as Router Solicitation (133), Router Advertisement (134), Neighbor Solicitation (135), Neighbor Advertisement (136), and Redirect (137). Without permitting these types, the router cannot resolve IPv6 neighbors or maintain IPv6 connectivity, as ND is fundamental to IPv6 operation.

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: "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?

Read the scenario before looking for a memorised answer.

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