- A
The OSPFv3 process has 'no ipv6 unicast-routing' enabled globally.
Why wrong: IPv6 unicast routing is required for OSPFv3 to function, but if it were disabled, the router would not even form adjacencies.
- B
The interface on R2 is configured as passive under OSPFv3.
Why wrong: A passive interface would still allow the router to form adjacency with R1, but it would not advertise the prefix of that interface, yet the loopback is not on that interface.
- C
The interface on R1 does not have an IPv6 address configured.
OSPFv3 requires an IPv6 address on the interface to advertise the connected prefix; without it, the router cannot originate a route for that link, and other routes may not be learned if the neighbor also lacks IPv6 addressing.
- D
The OSPFv3 process is configured with 'default-information originate always' but no default route exists.
Why wrong: This would affect only default route propagation, not the installation of OSPF routes.
Quick Answer
The answer is that the interface on R1 does not have an IPv6 address configured. In OSPFv3, an IPv6 address must be present on the interface for the router to advertise connected prefixes into the OSPFv3 database; without it, the router can form a neighbor adjacency (as seen with the FULL/DR state) but cannot install any OSPFv3 routes because there is no local IPv6 prefix to originate or receive. This scenario tests your understanding that OSPFv3 adjacency and route installation are separate processes—a common trap on the Cisco CCNP ENARSI 300-410 exam, where engineers mistakenly focus on network type or DR/BDR issues when the real problem is a missing IPv6 address on the interface. Remember the memory tip: “No IPv6 address, no OSPFv3 route—adjacency is just a handshake, not a route table update.”
300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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 OSPFv3 on a dual-stack network. Routers R1 and R2 are connected via a serial link. Both routers have OSPFv3 configured for IPv6. The engineer runs 'show ipv6 ospf neighbor' on R1 and sees R2 as FULL/DR. However, R1 cannot ping the IPv6 address of R2's loopback interface. 'show ipv6 route ospf' on R1 does not show any OSPF routes. What is the most likely cause?
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 interface on R1 does not have an IPv6 address configured.
In OSPFv3, the network type on serial interfaces defaults to POINT-TO-POINT, but if the interface is configured as BROADCAST (e.g., using 'ipv6 ospf network broadcast'), a DR/BDR election occurs. However, the adjacency is FULL, so the issue is not adjacency. The lack of OSPF routes in the routing table suggests that the routes are not being installed. One common reason is that OSPFv3 requires an IPv6 address on the interface, but the router may have an IPv4-only configuration or the interface may not have an IPv6 address. Another possibility is that the OSPFv3 process is not redistributing connected routes. The most likely cause is that the interface does not have an IPv6 address configured, which is required for OSPFv3 to advertise prefixes.
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 OSPFv3 process has 'no ipv6 unicast-routing' enabled globally.
Why it's wrong here
IPv6 unicast routing is required for OSPFv3 to function, but if it were disabled, the router would not even form adjacencies.
- ✗
The interface on R2 is configured as passive under OSPFv3.
Why it's wrong here
A passive interface would still allow the router to form adjacency with R1, but it would not advertise the prefix of that interface, yet the loopback is not on that interface.
- ✓
The interface on R1 does not have an IPv6 address configured.
Why this is correct
OSPFv3 requires an IPv6 address on the interface to advertise the connected prefix; without it, the router cannot originate a route for that link, and other routes may not be learned if the neighbor also lacks IPv6 addressing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The OSPFv3 process is configured with 'default-information originate always' but no default route exists.
Why it's wrong here
This would affect only default route propagation, not the installation of OSPF routes.
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?
OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The interface on R1 does not have an IPv6 address configured. — In OSPFv3, the network type on serial interfaces defaults to POINT-TO-POINT, but if the interface is configured as BROADCAST (e.g., using 'ipv6 ospf network broadcast'), a DR/BDR election occurs. However, the adjacency is FULL, so the issue is not adjacency. The lack of OSPF routes in the routing table suggests that the routes are not being installed. One common reason is that OSPFv3 requires an IPv6 address on the interface, but the router may have an IPv4-only configuration or the interface may not have an IPv6 address. Another possibility is that the OSPFv3 process is not redistributing connected routes. The most likely cause is that the interface does not have an IPv6 address configured, which is required for OSPFv3 to advertise prefixes.
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
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