- A
The router is not configured with a router ID
Why wrong: Without a router ID, OSPF would not form neighbors at all, so this cannot be the issue because neighbors are already FULL.
- B
The router has an ACL blocking inbound OSPF updates
An ACL that blocks OSPF protocol traffic (e.g., IP protocol 89) can prevent Type 3 LSAs from being received, causing inter-area routes to be missing while local adjacencies remain.
- C
The router is configured as an ABR but does not have a virtual-link configured
Why wrong: Virtual-links are only required if a non-backbone area is not directly connected to area 0; the scenario does not indicate that situation.
- D
The link-state database is corrupted
Why wrong: A corrupted LSDB would likely cause neighbor adjacencies to fail or routes to be inconsistent; given that neighbors are FULL, this is not the most likely cause.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 implementing OSPF on a router. All directly connected neighbors are listed with state FULL, but routes from another area are not appearing in the routing table. Which of the following 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 router has an ACL blocking inbound OSPF updates
Option B is correct because an ACL applied to the OSPF process or interface can filter inbound Type 3 LSAs (summary LSAs) from other areas. Even though neighbor adjacencies reach FULL state, the router will not install those inter-area routes into the routing table if the ACL blocks the LSA updates. This explains why directly connected neighbors are fine but routes from another area are missing.
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 router is not configured with a router ID
- ✓
The router has an ACL blocking inbound OSPF updates
Why this is correct
An ACL that blocks OSPF protocol traffic (e.g., IP protocol 89) can prevent Type 3 LSAs from being received, causing inter-area routes to be missing while local adjacencies remain.
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 router is configured as an ABR but does not have a virtual-link configured
Why it's wrong here
Virtual-links are only required if a non-backbone area is not directly connected to area 0; the scenario does not indicate that situation.
- ✗
The link-state database is corrupted
Why it's wrong here
A corrupted LSDB would likely cause neighbor adjacencies to fail or routes to be inconsistent; given that neighbors are FULL, this is not the most likely cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that FULL neighbor state guarantees all routes are learned, when in fact ACLs or distribute-lists can filter LSAs without affecting the neighbor relationship.
Trap categories for this question
Scenario analysis trap
Virtual-links are only required if a non-backbone area is not directly connected to area 0; the scenario does not indicate that situation.
Detailed technical explanation
How to think about this question
OSPF uses Type 3 LSAs to advertise prefixes between areas, generated by ABRs. An inbound ACL on the receiving router can filter these LSAs before they are processed by the SPF algorithm, preventing the routes from being installed in the RIB. In a real-world scenario, a misconfigured ACL might block OSPF multicast address 224.0.0.5/6 or specific LSA types, causing partial routing table issues while adjacencies remain up.
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.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router has an ACL blocking inbound OSPF updates — Option B is correct because an ACL applied to the OSPF process or interface can filter inbound Type 3 LSAs (summary LSAs) from other areas. Even though neighbor adjacencies reach FULL state, the router will not install those inter-area routes into the routing table if the ACL blocks the LSA updates. This explains why directly connected neighbors are fine but routes from another area are missing.
What should I do if I get this N10-009 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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Last reviewed: Jun 11, 2026
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.
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