- A
The backup link has a lower OSPF cost than the primary link.
Why wrong: If the backup link has a lower cost, it would be preferred. However, the question states traffic is going over the backup link, so the primary should have higher cost, not lower.
- B
The OSPF cost of the primary link is higher than the backup link.
Correct. The primary link has a higher OSPF cost than the backup, causing the router to prefer the backup link.
- C
The reference bandwidth is set to 1 Gbps on the primary router.
Why wrong: Setting the reference bandwidth to 1 Gbps would make the primary link cost 1 and the backup cost 10, so the primary would be preferred. That does not match the issue.
- D
The bandwidth command is misconfigured on the backup interface.
Why wrong: Misconfiguring bandwidth on the backup interface could affect its cost, but the problem is the primary link not being preferred, so this is not the most likely cause.
N10-009 Networking Concepts Practice Question
This N10-009 practice question tests your understanding of networking concepts. 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 configures OSPF on two routers with a primary link (1 Gbps) and a backup link (100 Mbps). The engineer expects traffic to always use the primary link unless it fails, but the router is sending traffic over the backup link. 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.
Clue:
"primary"Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Clue:
"always"Why it matters: Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
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 OSPF cost of the primary link is higher than the backup link.
OSPF determines the best path using cost, which is calculated as (reference bandwidth) / (interface bandwidth). By default, the reference bandwidth is 100 Mbps. For a 1 Gbps link, the cost is 0.1, which is rounded up to 1. For a 100 Mbps link, the cost is exactly 1. This would normally result in equal-cost paths and load balancing, not preferring the backup link. However, if the router is sending traffic over the backup link, it indicates that the primary link has a higher OSPF cost than the backup. This can happen if the bandwidth statement on the primary interface is misconfigured (e.g., set to 100 Mbps) or if a manual cost is set higher. Thus, the most likely cause is that the primary link’s OSPF cost is higher than expected, causing the router to prefer the backup link.
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 backup link has a lower OSPF cost than the primary link.
Why it's wrong here
If the backup link has a lower cost, it would be preferred. However, the question states traffic is going over the backup link, so the primary should have higher cost, not lower.
When this WOULD be correct
In a scenario where the question asks about a routing protocol that does not consider bandwidth by default, such as RIP, and the engineer expects load balancing across equal-cost paths, option A might be correct if it refers to a misconfiguration of load balancing.
- ✓
The OSPF cost of the primary link is higher than the backup link.
Why this is correct
Correct. The primary link has a higher OSPF cost than the backup, causing the router to prefer the backup link.
Clue confirmation
The clue words "most likely", "primary", "always" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The reference bandwidth is set to 1 Gbps on the primary router.
Why it's wrong here
Setting the reference bandwidth to 1 Gbps would make the primary link cost 1 and the backup cost 10, so the primary would be preferred. That does not match the issue.
When this WOULD be correct
In a question where the engineer misconfigured OSPF network type (e.g., point-to-point vs broadcast) causing adjacency issues, selecting the correct network type would be the answer.
- ✗
The bandwidth command is misconfigured on the backup interface.
Why it's wrong here
Misconfiguring bandwidth on the backup interface could affect its cost, but the problem is the primary link not being preferred, so this is not the most likely cause.
When this WOULD be correct
If the question were about a different routing protocol or scenario where the backup link is preferred due to administrative distance or metric issues, option D might be correct. For example, if the backup link had a lower OSPF cost due to misconfiguration.
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 OSPF cost of the primary link is higher than the backup link.Correct answer▾
Why this is correct
Correct. The primary link has a higher OSPF cost than the backup, causing the router to prefer the backup link.
✗The backup link has a lower OSPF cost than the primary link.Wrong answer — click to see why▾
Why this is wrong here
Option A is not defined in the question, so it cannot be analyzed. The correct answer is B, which likely refers to OSPF cost calculation based on bandwidth, where the backup link may have a lower cost due to incorrect cost settings or reference bandwidth.
★ When this WOULD be the correct answer
In a scenario where the question asks about a routing protocol that does not consider bandwidth by default, such as RIP, and the engineer expects load balancing across equal-cost paths, option A might be correct if it refers to a misconfiguration of load balancing.
Why candidates choose this
Candidates might choose an undefined option out of confusion or because they think any answer could be correct without proper analysis, but this is not a valid exam strategy.
✗The reference bandwidth is set to 1 Gbps on the primary router.Wrong answer — click to see why▾
Why this is wrong here
Option C is not a standard OSPF configuration parameter; OSPF uses cost based on bandwidth, and the issue described is likely due to incorrect cost settings, not an option like 'C'.
★ When this WOULD be the correct answer
In a question where the engineer misconfigured OSPF network type (e.g., point-to-point vs broadcast) causing adjacency issues, selecting the correct network type would be the answer.
Why candidates choose this
Candidates might confuse OSPF configuration options or misremember a specific parameter, leading them to choose a plausible-sounding but incorrect option.
✗The bandwidth command is misconfigured on the backup interface.Wrong answer — click to see why▾
Why this is wrong here
Option D is not provided in the question, so it cannot be analyzed. The correct answer is B, which likely refers to a misconfiguration such as incorrect OSPF cost settings on the primary link.
★ When this WOULD be the correct answer
If the question were about a different routing protocol or scenario where the backup link is preferred due to administrative distance or metric issues, option D might be correct. For example, if the backup link had a lower OSPF cost due to misconfiguration.
Why candidates choose this
Candidates might choose D if they mistakenly believe that the backup link is being used because of a failover mechanism or load balancing, rather than identifying the specific OSPF cost misconfiguration.
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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the misconception that OSPF prefers higher bandwidth links automatically, but the trap is that OSPF's default cost calculation makes any link at 100 Mbps or faster have the same cost of 1, causing equal-cost multipath (ECMP) instead of primary/backup behavior.
Detailed technical explanation
How to think about this question
OSPF's cost formula (reference bandwidth / interface bandwidth) defaults to 10^8 bps, meaning any link faster than 100 Mbps also gets a cost of 1 unless the reference bandwidth is manually increased (e.g., 'auto-cost reference-bandwidth 10000' for 10 Gbps links). In real-world networks, this equal-cost scenario often occurs when mixing Gigabit and Fast Ethernet links without adjusting the reference bandwidth, leading to unintended ECMP.
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.
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.
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FAQ
Questions learners often ask
What does this N10-009 question test?
Networking Concepts — This question tests Networking Concepts — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The OSPF cost of the primary link is higher than the backup link. — OSPF determines the best path using cost, which is calculated as (reference bandwidth) / (interface bandwidth). By default, the reference bandwidth is 100 Mbps. For a 1 Gbps link, the cost is 0.1, which is rounded up to 1. For a 100 Mbps link, the cost is exactly 1. This would normally result in equal-cost paths and load balancing, not preferring the backup link. However, if the router is sending traffic over the backup link, it indicates that the primary link has a higher OSPF cost than the backup. This can happen if the bandwidth statement on the primary interface is misconfigured (e.g., set to 100 Mbps) or if a manual cost is set higher. Thus, the most likely cause is that the primary link’s OSPF cost is higher than expected, causing the router to prefer the backup link.
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", "primary", "always". 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 30, 2026
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