- A
The virtual link is configured only on one router.
Why wrong: If the virtual link is configured on only one router, the adjacency cannot form. Since the stem states the virtual link is up, this cannot be the cause.
- B
The transit area is a stub area.
Correct. A virtual link through a stub area is invalid in OSPF. The transit area must be a standard area or an NSSA, not a stub area.
- C
The OSPF process is configured with 'no-virtual-link' command.
Why wrong: The 'no-virtual-link' command is not a standard OSPF command; it is likely a distractor. If the virtual link is up, this command has not been applied.
- D
The router IDs are not reachable via the transit area.
Why wrong: If router IDs were not reachable via the transit area, the virtual link would not establish. Since the virtual link is up, reachability is confirmed.
300-410 OSPF virtual link 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. A key principle to apply: oSPF virtual link. 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.
An engineer configures OSPFv2 with a virtual link to connect a non-backbone area to area 0. The virtual link is up, but routes from the non-backbone area are not being advertised into area 0. Which is the most likely explanation?
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 transit area is a stub area.
In OSPF, virtual links cannot be configured through stub areas. If the transit area is a stub area, the virtual link will not function correctly, and routes from the non-backbone area will not be advertised into area 0. Even if the virtual link appears to be up (e.g., due to misconfiguration or software bug), route propagation is blocked because stub areas do not allow virtual links or type 5 LSAs. Therefore, the most likely explanation is that the transit area is a stub area.
Key principle: OSPF virtual link
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 virtual link is configured only on one router.
Why it's wrong here
If the virtual link is configured on only one router, the adjacency cannot form. Since the stem states the virtual link is up, this cannot be the cause.
- ✓
The transit area is a stub area.
- ✗
The OSPF process is configured with 'no-virtual-link' command.
Why it's wrong here
The 'no-virtual-link' command is not a standard OSPF command; it is likely a distractor. If the virtual link is up, this command has not been applied.
- ✗
The router IDs are not reachable via the transit area.
Why it's wrong here
If router IDs were not reachable via the transit area, the virtual link would not establish. Since the virtual link is up, reachability is confirmed.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Candidates often overlook that virtual links cannot traverse stub areas. This question tests that rule.
Trap categories for this question
Command / output trap
The 'no-virtual-link' command is not a standard OSPF command; it is likely a distractor. If the virtual link is up, this command has not been applied.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- OSPF virtual link
- Transit area
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
OSPF virtual link
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.
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.
What to study next
Got this wrong? Here's your next step.
Review oSPF virtual link, then practise related 300-410 questions on the same topic to reinforce the concept.
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OSPF Troubleshooting (v2/v3) — study guide chapter
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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 virtual link.
What is the correct answer to this question?
The correct answer is: The transit area is a stub area. — In OSPF, virtual links cannot be configured through stub areas. If the transit area is a stub area, the virtual link will not function correctly, and routes from the non-backbone area will not be advertised into area 0. Even if the virtual link appears to be up (e.g., due to misconfiguration or software bug), route propagation is blocked because stub areas do not allow virtual links or type 5 LSAs. Therefore, the most likely explanation is that the transit area is a stub area.
What should I do if I get this 300-410 question wrong?
Review oSPF virtual link, then practise related 300-410 questions on the same topic to reinforce the concept.
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 virtual link
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Last reviewed: Jun 18, 2026
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