Quick Answer
The answer is 100 Mbps, which is the default OSPF reference bandwidth used for cost calculation in Cisco IOS. OSPF determines interface cost by dividing this reference bandwidth by the actual interface bandwidth, so a FastEthernet link at 100 Mbps yields a cost of 1—the lowest possible value. This default was set when FastEthernet represented cutting-edge speed, but on the ENCOR 350-401 exam, you must recognize that modern networks with GigabitEthernet or 10 GigabitEthernet would all share the same minimum cost of 1 unless you adjust the reference using the `auto-cost reference-bandwidth` command. A common trap is assuming OSPF automatically scales for faster links, but without manual configuration, the cost calculation becomes meaningless for high-speed interfaces. To remember this, think of the default as “100 for FastEthernet”—if you see a cost of 1 on a Gigabit link, the reference bandwidth hasn’t been tuned.
This 350-401 practice question tests your understanding of ospf. 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 for cost calculation in Cisco IOS?
100 Mbps
The default reference bandwidth is 100 Mbps, as defined in the OSPF specification.
1000 Mbps
Why wrong: 1000 Mbps is not the default; it can be changed with the 'auto-cost reference-bandwidth' command.
10 Mbps
Why wrong: 10 Mbps is the bandwidth of a link, not the reference bandwidth.
1 Mbps
Why wrong: 1 Mbps is too low; the default is 100 Mbps.
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
In Cisco IOS, the default OSPF reference bandwidth is 100 Mbps. OSPF calculates the cost of an interface as the reference bandwidth divided by the interface bandwidth. With the default reference of 100 Mbps, a FastEthernet (100 Mbps) interface gets a cost of 1, which is the minimum cost. This default was established when FastEthernet was considered high-speed, but it can be changed using the 'auto-cost reference-bandwidth' command to accommodate faster links like GigabitEthernet.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
100 Mbps
Why this is correct
The default reference bandwidth is 100 Mbps, as defined in the OSPF specification.
Related concept
Read the scenario before looking for a memorised answer.
1000 Mbps
Why it's wrong here
1000 Mbps is not the default; it can be changed with the 'auto-cost reference-bandwidth' command.
10 Mbps
Why it's wrong here
10 Mbps is the bandwidth of a link, not the reference bandwidth.
1 Mbps
Why it's wrong here
1 Mbps is too low; the default is 100 Mbps.
Common exam traps
Cisco often tests the default OSPF reference bandwidth as 100 Mbps, and the trap here is that candidates confuse it with the actual interface bandwidth (e.g., 10 Mbps for Ethernet) or assume it matches the fastest common link speed (e.g., 1000 Mbps for GigabitEthernet).
Trap categories for this question
Command / output trap
1000 Mbps is not the default; it can be changed with the 'auto-cost reference-bandwidth' command.
Detailed technical explanation
OSPF cost is calculated as reference_bandwidth / interface_bandwidth, where the default reference is 100 Mbps. On a GigabitEthernet interface (1000 Mbps), this yields a cost of 0.1, which is truncated to 1, making all links from 100 Mbps to 10 Gbps appear equal. To fix this, network engineers use 'auto-cost reference-bandwidth 10000' to set the reference to 10 Gbps, ensuring accurate cost differentiation on high-speed links. This command is global and affects all OSPF processes on the router.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
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FAQ
OSPF — This question tests OSPF — Read the scenario before looking for a memorised answer..
The correct answer is: 100 Mbps — In Cisco IOS, the default OSPF reference bandwidth is 100 Mbps. OSPF calculates the cost of an interface as the reference bandwidth divided by the interface bandwidth. With the default reference of 100 Mbps, a FastEthernet (100 Mbps) interface gets a cost of 1, which is the minimum cost. This default was established when FastEthernet was considered high-speed, but it can be changed using the 'auto-cost reference-bandwidth' command to accommodate faster links like GigabitEthernet.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This 350-401 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 350-401 exam.
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