mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an IPv6 routing issue on a…
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?
⚠ Common exam trap
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.
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 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.
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.
- ✗
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
IPv6 Next Hop Resolution
IPv6 Next Hop Resolution is the process a router uses to determine the next router or destination device to which a packet should be forwarded, based on the destination IPv6 address and the routing table.
Key term
IPv6 Routing OSPFv3
OSPFv3 is a routing protocol that lets routers share information about how to reach destinations in an IPv6 network, updating routes automatically as the network changes.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.