The answer is that the ACL is dropping OSPFv3 packets from the neighbor because they contain a different area ID than what the ACL permits. When you use the `ospfv3` keyword in an IPv6 ACL, you can optionally match on the OSPFv3 area ID field within the packet header. If the ACL is configured to permit only a specific area ID, any OSPFv3 packet—including Hello packets—carrying a different area ID will be implicitly denied, breaking adjacency formation and preventing route learning. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPv6 ACL filtering interacts with OSPFv3 area boundaries; a common trap is assuming the ACL only filters routing updates rather than all OSPFv3 control traffic. Remember that OSPFv3 Hello packets carry the originating interface’s area ID, so an inbound ACL that restricts area IDs effectively blocks adjacencies across different areas. A useful memory tip: “Area ID in the ACL means area ID in the Hello—match or miss the neighbor.”
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 an IPv6 routing issue where a router is not learning routes from an OSPFv3 neighbor. The engineer checks the interface and finds an inbound IPv6 ACL that permits only OSPFv3 packets with a specific area ID in the packet. The ACL is using the 'ospfv3' keyword to match packets. The engineer also notices that the OSPFv3 neighbor is in a different area. What is the most likely cause of the route learning 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The ACL is dropping OSPFv3 packets from the neighbor because they contain a different area ID than what the ACL permits.
The ACL uses the 'ospfv3' keyword to match OSPFv3 packets and permits only those with a specific area ID. Since the neighbor is in a different area, its OSPFv3 packets contain a different area ID in the OSPFv3 header, causing the ACL to deny them. This prevents the router from receiving Hello packets and establishing adjacency, so routes are not learned.
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 is dropping OSPFv3 packets from the neighbor because they contain a different area ID than what the ACL permits.
Why this is correct
Correct because the ACL permits only packets with a specific area ID, and the neighbor is in a different area, so its packets are dropped.
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 OSPFv3 process is not configured with the correct router ID.
Why it's wrong here
Incorrect because the neighbor relationship is not forming due to the ACL, not the router ID.
✗
The interface is not enabled for OSPFv3.
Why it's wrong here
Incorrect because the neighbor is sending packets, indicating OSPFv3 is enabled on the interface.
✗
The ACL is applied outbound, blocking the OSPFv3 packets from being sent.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the nuance that the 'ospfv3' ACL keyword can match not only the protocol but also the area ID field, and candidates mistakenly assume the ACL only matches the protocol type (OSPFv3) without considering the area ID filter.
Detailed technical explanation
How to think about this question
OSPFv3 uses the IPv6 Next Header value 89 for OSPF, and the 'ospfv3' keyword in an IPv6 ACL matches this protocol. The area ID is carried in the OSPFv3 packet header (a 32-bit field); when an ACL permits only a specific area ID, it inspects this field using the 'area-id' parameter. In real-world scenarios, misconfigured ACLs with area ID filtering can silently break OSPFv3 adjacency across area boundaries, leading to partial routing tables that are difficult to diagnose without packet capture.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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 is dropping OSPFv3 packets from the neighbor because they contain a different area ID than what the ACL permits. — The ACL uses the 'ospfv3' keyword to match OSPFv3 packets and permits only those with a specific area ID. Since the neighbor is in a different area, its OSPFv3 packets contain a different area ID in the OSPFv3 header, causing the ACL to deny them. This prevents the router from receiving Hello packets and establishing adjacency, so routes are not learned.
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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