Quick Answer
The answer is 100 Mbps. This is the default OSPF reference bandwidth used in the cost calculation formula on Cisco IOS, where the cost of an interface is derived by dividing the reference bandwidth (100,000,000 bps) by the interface bandwidth in bps. The technical concept behind this is that OSPF uses cost as its metric to determine the best path, and the reference bandwidth serves as a fixed baseline to ensure consistent cost values across different link speeds. On the Cisco CCNP ENARSI 300-410 exam, this topic tests your understanding of OSPF metric fundamentals and often appears in questions about path selection or troubleshooting suboptimal routing. A common trap is forgetting that the default reference bandwidth of 100 Mbps can make high-speed interfaces like Gigabit Ethernet appear to have the same cost as Fast Ethernet, leading to equal-cost paths where none should exist. A helpful memory tip is to think of the default as "100 for 100"—100 Mbps for the reference, and the cost formula uses 100,000,000 divided by the interface speed.
This 300-410 practice question tests your understanding of route summarization. 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.
What is the default OSPF reference bandwidth used in the cost calculation formula on Cisco IOS?
10 Mbps
Why wrong: 10 Mbps is not the default; the default is 100 Mbps.
100 Mbps
Cisco IOS uses 100 Mbps as the reference bandwidth by default.
1000 Mbps
Why wrong: 1000 Mbps is the default for newer routers but not for standard Cisco IOS.
1 Gbps
Why wrong: 1 Gbps is not the default; it can be configured manually with the auto-cost reference-bandwidth command.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
100 Mbps
The default reference bandwidth is 100 Mbps, meaning the cost is calculated as 100,000,000 / interface bandwidth in bps.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
10 Mbps
Why it's wrong here
10 Mbps is not the default; the default is 100 Mbps.
100 Mbps
Why this is correct
Cisco IOS uses 100 Mbps as the reference bandwidth by default.
Related concept
OSPF neighbours must agree on key parameters.
1000 Mbps
Why it's wrong here
1000 Mbps is the default for newer routers but not for standard Cisco IOS.
1 Gbps
Why it's wrong here
1 Gbps is not the default; it can be configured manually with the auto-cost reference-bandwidth command.
Common exam traps
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
1 Gbps is not the default; it can be configured manually with the auto-cost reference-bandwidth command.
Detailed technical explanation
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
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
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Route Summarization — study guide chapter
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FAQ
Route Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..
The correct answer is: 100 Mbps — The default reference bandwidth is 100 Mbps, meaning the cost is calculated as 100,000,000 / interface bandwidth in bps.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
This 300-410 practice question is part of Courseiva's free Cisco 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 300-410 exam.
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