easyMultiple Choice
350-401 Practice Question: The default OSPF reference bandwidth used for…
What is the default OSPF reference bandwidth used for cost calculation in Cisco IOS?
⚠ Common exam trap
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).
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
✓
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
100 Mbps
Why this is correct
100 Mbps is the OSPF default reference bandwidth defined in RFC 2328, and it is used in the formula cost = reference-bandwidth / interface-bandwidth. With this default, a 100 Mbps interface has a cost of 1, and any interface faster than 100 Mbps also receives a cost of 1 because OSPF costs are integer values; this is why network administrators often raise the reference bandwidth in modern high-speed networks.
- ✗
1000 Mbps
Why it's wrong here
1000 Mbps (1 Gbps) is not the OSPF default; it is a commonly configured value set via the 'auto-cost reference-bandwidth' command. If an administrator sets the reference bandwidth to 1000 Mbps, then a 1 Gbps link gets a cost of 1, but the cost for a 10 Gbps link would be 0.1, which OSPF rounds up to 1, making those links equal unless an even higher reference is configured. This option is wrong because the question asks for the default value, and 1000 Mbps is an intentional administrative override, not the specification default.
- ✗
10 Mbps
Why it's wrong here
10 Mbps is an actual interface bandwidth often seen on legacy Ethernet links, but it is not the OSPF reference bandwidth. The reference bandwidth is a fixed constant in the OSPF cost equation, independent of any single link's bandwidth. If the reference were 10 Mbps, then only a 10 Mbps link would have a cost of 1, and all faster links (100 Mbps, 1 Gbps, etc.) would also have a cost of 1 due to integer rounding, making the metric useless for path selection.
- ✗
1 Mbps
Why it's wrong here
1 Mbps is far below the OSPF default and would render the cost metric practically meaningless, because virtually every modern interface would have a calculated cost below 1, and OSPF would round each to a cost of 1. This would eliminate any preference for higher-bandwidth links, defeating OSPF's purpose of choosing the best path based on interface speed. The RFC specifically selected 100 Mbps as a baseline that provides a reasonable dynamic range for typical network link speeds of that era.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.