The answer is B because OSPF equal cost load balancing is occurring, not a routing failure. OSPF calculates path cost using the formula 10^8 / interface bandwidth, so both a 1 Gbps link (cost 1) and a 100 Mbps link (cost 1) yield identical costs, causing the router to treat them as equal-cost paths and distribute traffic across both. On the CompTIA Network+ N10-009 exam, this question tests your understanding that OSPF does not inherently prefer higher bandwidth—it only prefers lower cost, and when costs match, it load-balances. A common trap is assuming OSPF automatically favors faster links, but the cost formula caps at 10^8, making anything above 100 Mbps equal to a cost of 1. To remember: OSPF sees 1 Gbps and 100 Mbps as the same cost, so it splits traffic—think “equal cost, equal traffic.”
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.
Why wrong: A lower cost makes a route more preferred. The backup link has a higher cost, so this would not cause it to be preferred.
B
B
If the primary link's cost is higher (e.g., due to a misconfigured interface bandwidth), OSPF will choose the lower-cost backup link.
C
C
Why wrong: OSPF uses cost, not hop count. Hop count is used by RIP.
D
D
Why wrong: Being in a different OSPF area does not inherently affect cost calculations unless the route is inter-area, but the scenario describes two directly connected links.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
B
OSPF uses cost to determine the best path, calculated as 10^8 / interface bandwidth. A 1 Gbps link has a cost of 1 (100,000,000 / 1,000,000,000), while a 100 Mbps link has a cost of 1 (100,000,000 / 100,000,000). Since both costs are equal, OSPF will load-balance traffic across both links instead of preferring the primary link, which is why traffic is seen on 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.
✗
A
Why it's wrong here
A lower cost makes a route more preferred. The backup link has a higher cost, so this would not cause it to be preferred.
✓
B
Why this is correct
If the primary link's cost is higher (e.g., due to a misconfigured interface bandwidth), OSPF will choose the lower-cost 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.
✗
C
Why it's wrong here
OSPF uses cost, not hop count. Hop count is used by RIP.
✗
D
Why it's wrong here
Being in a different OSPF area does not inherently affect cost calculations unless the route is inter-area, but the scenario describes two directly connected links.
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.
Trap categories for this question
Scenario analysis trap
Being in a different OSPF area does not inherently affect cost calculations unless the route is inter-area, but the scenario describes two directly connected links.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
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: B — OSPF uses cost to determine the best path, calculated as 10^8 / interface bandwidth. A 1 Gbps link has a cost of 1 (100,000,000 / 1,000,000,000), while a 100 Mbps link has a cost of 1 (100,000,000 / 100,000,000). Since both costs are equal, OSPF will load-balance traffic across both links instead of preferring the primary link, which is why traffic is seen on 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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