The answer is that the inbound IPv6 ACL is filtering the OSPFv3 routes by prefix length, denying the /48 routes while permitting only /64 prefixes. This occurs because IPv6 ACLs applied to OSPFv3 interfaces can match on the prefix length field within the route advertisement, not just the destination network. When the ACL permits only /64 prefixes, any route with a different prefix length—such as a /48—is implicitly denied and thus not installed in the routing table. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPv6 ACLs interact with OSPFv3 route filtering, a common trap where engineers mistakenly assume ACLs only filter data-plane traffic rather than control-plane routing updates. A key memory tip is “prefix length is the gatekeeper”—the ACL checks the length before the prefix itself, so if the length doesn’t match, the route is dropped regardless of the network address.
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 on a router that is receiving OSPFv3 routes from multiple neighbors. The engineer notices that some routes are missing from the routing table. The engineer checks the interface configuration and finds an inbound IPv6 ACL applied to the interface that permits only specific prefixes. The engineer also notices that the missing routes are from a neighbor that is sending routes with a prefix length of /48, while the ACL permits only /64 prefixes. What is the most likely cause of the missing routes?
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 filtering the OSPFv3 routes based on prefix length, and the /48 routes are not permitted.
The inbound IPv6 ACL on the interface is filtering OSPFv3 routes based on prefix length. Since the ACL permits only /64 prefixes, any routes with a /48 prefix length are denied and not installed in the routing table. This is the most likely cause of the missing routes.
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 filtering the OSPFv3 routes based on prefix length, and the /48 routes are not permitted.
Why this is correct
Correct because the ACL permits only /64 prefixes, so /48 routes are dropped, preventing them from being installed in the routing table.
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 neighbor relationship is down due to a mismatched area ID.
Why it's wrong here
Incorrect because the neighbor is sending routes, indicating the adjacency is up.
✗
The router has a route-map that is denying the /48 routes before they are installed.
Why it's wrong here
Incorrect because the symptom is specifically related to the ACL on the interface, not a route-map.
✗
The IPv6 ACL is applied outbound, blocking the routes from being sent.
Why it's wrong here
Incorrect because the ACL is applied inbound, as stated in the scenario.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may overlook that an inbound ACL on an interface can filter OSPFv3 route updates based on prefix length, assuming ACLs only filter data-plane traffic, not routing protocol updates.
Trap categories for this question
Scenario analysis trap
Incorrect because the ACL is applied inbound, as stated in the scenario.
Detailed technical explanation
How to think about this question
OSPFv3 uses link-local addresses for neighbor adjacencies and can carry IPv6 prefixes in Type-3 (Inter-Area Prefix) and Type-5 (AS-External) LSAs. An inbound IPv6 ACL on an interface filters all IPv6 traffic, including OSPFv3 packets, which can prevent specific prefixes from being processed and installed in the RIB. In real-world scenarios, engineers must ensure ACLs permit the OSPFv3 protocol itself (e.g., permit 89) and the desired prefix lengths, or use prefix-list filtering under the OSPFv3 process instead.
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 filtering the OSPFv3 routes based on prefix length, and the /48 routes are not permitted. — The inbound IPv6 ACL on the interface is filtering OSPFv3 routes based on prefix length. Since the ACL permits only /64 prefixes, any routes with a /48 prefix length are denied and not installed in the routing table. This is the most likely cause of the missing routes.
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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