N10-009 Network Implementation Practice Question
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?
⚠ Common exam trap
The N10-009 exam 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.
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
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.
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
Why it's wrong here
The OSPF router ID is a fundamental identifier used in the neighbor discovery and adjacency formation process. Without a unique router ID, OSPF processes would fail to initialize, and routers would be unable to exchange Hello packets or establish any neighbor adjacencies whatsoever. Since the problem states that neighbors are already in a FULL state, indicating successful adjacency formation, the absence of a router ID cannot be the cause of missing inter-area routes.
When this WOULD be correct
In a scenario where OSPF neighbors fail to reach FULL state and the router cannot establish adjacencies, a missing router ID would be the likely cause because OSPF requires a unique router ID to form neighbor relationships.
- ✓
The router has an ACL blocking inbound OSPF updates
Why this is correct
An Access Control List (ACL) configured to deny inbound OSPF protocol traffic (IP protocol 89) would specifically prevent the reception of Link-State Advertisements (LSAs) from other areas. While OSPF neighbor adjacencies, which primarily rely on Hello packets, might still form and reach a FULL state within the local area, the router would fail to populate its routing table with inter-area routes (e.g., those advertised via Type 3 LSAs). This scenario perfectly explains why local adjacencies are full but routes to other areas are missing.
- ✗
The router is configured as an ABR but does not have a virtual-link configured
Why it's wrong here
A virtual link in OSPF is specifically used to connect a non-backbone area to Area 0 when it doesn't have a direct physical connection, or to connect a disconnected Area 0 segment. If a router is an Area Border Router (ABR) and its non-backbone areas are already directly connected to Area 0, a virtual link is not necessary for inter-area routing. The absence of a virtual link would only be problematic if the network topology explicitly required it, which is not implied by the problem statement, and it wouldn't inherently prevent all inter-area routes if the ABR itself is properly connected to Area 0.
- ✗
The link-state database is corrupted
Why it's wrong here
A corrupted Link-State Database (LSDB) would typically manifest as severe OSPF instability, potentially leading to repeated neighbor flapping, inconsistent routing information, or even a complete failure to form adjacencies. While a corrupted LSDB could result in missing routes, the fact that neighbors are explicitly stated to be in a FULL state suggests a more specific issue than general database corruption. A healthy LSDB is crucial for maintaining stable adjacencies and accurate route calculation, making widespread corruption inconsistent with stable FULL adjacencies.
When this WOULD be correct
In a question where OSPF neighbors are stuck in EXSTART or EXCHANGE state, or where routes are missing due to LSDB corruption (e.g., after a power failure), this option would be correct.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓The router has an ACL blocking inbound OSPF updatesCorrect answer▾
Why this is correct
An Access Control List (ACL) configured to deny inbound OSPF protocol traffic (IP protocol 89) would specifically prevent the reception of Link-State Advertisements (LSAs) from other areas. While OSPF neighbor adjacencies, which primarily rely on Hello packets, might still form and reach a FULL state within the local area, the router would fail to populate its routing table with inter-area routes (e.g., those advertised via Type 3 LSAs). This scenario perfectly explains why local adjacencies are full but routes to other areas are missing.
✗The router is not configured with a router IDWrong answer — click to see why▾
Why this is wrong here
OSPF can dynamically assign a router ID using the highest loopback or physical IP address if none is configured, so missing a router ID would not prevent routes from another area from appearing.
★ When this WOULD be the correct answer
In a scenario where OSPF neighbors fail to reach FULL state and the router cannot establish adjacencies, a missing router ID would be the likely cause because OSPF requires a unique router ID to form neighbor relationships.
Why candidates choose this
Candidates may think a router ID is mandatory for OSPF to function, but they overlook that OSPF can auto-select one, and the issue here is about inter-area routes, not neighbor formation.
✗The link-state database is corruptedWrong answer — click to see why▾
Why this is wrong here
A corrupted link-state database would typically cause OSPF neighbor states to not reach FULL or cause routing loops, not specifically prevent routes from another area from appearing while neighbors are FULL.
★ When this WOULD be the correct answer
In a question where OSPF neighbors are stuck in EXSTART or EXCHANGE state, or where routes are missing due to LSDB corruption (e.g., after a power failure), this option would be correct.
Why candidates choose this
Candidates may think that any missing route is due to database corruption, especially if they've studied LSDB issues, but here the FULL neighbor state indicates LSDB synchronization is complete.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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
Physical Network Infrastructure
Key term
Routing table
A routing table is a data set stored in a router or host that contains information about network paths and is used to determine where to forward data packets.
Key term
ACL
An Access Control List is a set of rules that determines who or what can access specific network resources or data.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.