- A
The ACL is applied inbound on R1 and does not permit ICMPv6 echo replies from R2.
Correct because ICMPv6 echo replies are sourced from the destination address (R2) and must be permitted inbound on R1 for the ping to succeed.
- B
The ACL is applied outbound on R1 and blocks the echo request.
Why wrong: Incorrect because the ACL is applied inbound, not outbound.
- C
IPv6 unicast-routing is not enabled on R1.
Why wrong: Incorrect because R1 can ping its own address, indicating IPv6 is operational; unicast-routing is not required for direct link pings.
- D
The ACL is missing a permit statement for IPv6 neighbor discovery (ND) messages.
Why wrong: Incorrect because ND messages are needed for address resolution, but the symptom is a ping failure after ND has likely succeeded (R1 can ping its own address, implying layer 2 is up).
Quick Answer
The answer is that the inbound IPv6 ACL on R1 is blocking the ICMPv6 echo reply from R2. For a ping to succeed, the echo request must reach the destination and the echo reply must return to the source; if the ACL only permits echo requests from a specific source, it will drop the reply sourced from R2’s address. This scenario tests your understanding of stateful versus stateless ACL behavior in IPv6—a common trap on the Cisco CCNP ENARSI 300-410 exam where engineers mistakenly permit only the request direction. Remember that IPv6 ACLs are stateless, so each packet is evaluated independently; the reply is a different packet with a different source address. A useful memory tip is “request out, reply in”—both directions must be explicitly allowed for ICMP echo.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
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.
A network engineer is troubleshooting a connectivity issue between two routers R1 and R2 connected via GigabitEthernet0/0. The engineer notices that R1 can ping its own IPv6 address 2001:db8:1::1/64, but cannot ping R2's interface address 2001:db8:1::2/64. The output of 'show ipv6 interface GigabitEthernet0/0' on R1 indicates that IPv6 is enabled and the interface is up/up. The engineer checks the access list applied to the interface and sees an inbound IPv6 ACL that permits only ICMPv6 echo requests from a specific source. What is the most likely cause of the ping failure?
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 is applied inbound on R1 and does not permit ICMPv6 echo replies from R2.
The inbound ACL on R1 is blocking the ICMPv6 echo reply from R2, because the ACL only permits echo requests from a specific source, not replies. For a successful ping, both the echo request and echo reply must be permitted. The reply is sourced from R2's address, which is not matched by the permit statement.
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 is applied inbound on R1 and does not permit ICMPv6 echo replies from R2.
Why this is correct
Correct because ICMPv6 echo replies are sourced from the destination address (R2) and must be permitted inbound on R1 for the ping to succeed.
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 is applied outbound on R1 and blocks the echo request.
Why it's wrong here
Incorrect because the ACL is applied inbound, not outbound.
- ✗
IPv6 unicast-routing is not enabled on R1.
Why it's wrong here
Incorrect because R1 can ping its own address, indicating IPv6 is operational; unicast-routing is not required for direct link pings.
- ✗
The ACL is missing a permit statement for IPv6 neighbor discovery (ND) messages.
Why it's wrong here
Incorrect because ND messages are needed for address resolution, but the symptom is a ping failure after ND has likely succeeded (R1 can ping its own address, implying layer 2 is up).
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.
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.
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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 — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The ACL is applied inbound on R1 and does not permit ICMPv6 echo replies from R2. — The inbound ACL on R1 is blocking the ICMPv6 echo reply from R2, because the ACL only permits echo requests from a specific source, not replies. For a successful ping, both the echo request and echo reply must be permitted. The reply is sourced from R2's address, which is not matched by the permit statement.
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.
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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.
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