easyMultiple Choice
300-410 Practice Question: The default OSPF reference bandwidth used in the…
What is the default OSPF reference bandwidth used in the cost calculation formula on Cisco IOS?
⚠ Common exam trap
The trap is confusing the OSPF default reference bandwidth (100 Mbps) with the common manually configured value (1000 Mbps or higher) — candidates who have seen 'auto-cost reference-bandwidth 1000' in labs often pick 1000 Mbps as the default.
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
Cisco IOS uses a default OSPF reference bandwidth of 100 Mbps (10^8) in the cost formula cost = reference bandwidth / interface bandwidth. This default dates back to when Fast Ethernet was the fastest common link, and it means any interface 100 Mbps or faster yields a cost of 1 unless the reference bandwidth is changed.
Answer analysis
Option-by-option breakdown
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 corresponds to the original OSPF specification's reference value, not the Cisco IOS default, which was raised to 100 Mbps. It is tempting because early OSPF documentation and some vendor implementations cite 10 Mbps, but IOS computes cost as 100,000,000 divided by bandwidth unless changed.
- ✓
100 Mbps
Why this is correct
Cisco IOS defaults the OSPF reference bandwidth to 100 Mbps, so interface cost equals 100,000,000 divided by interface bandwidth in bits per second. This default makes any link of 100 Mbps or faster cost 1 unless reference bandwidth is raised.
- ✗
1000 Mbps
Why it's wrong here
1000 Mbps is not the IOS default; it is the value administrators commonly set when raising the reference for gigabit-and-faster links. The default remains 100 Mbps, so cost is calculated as 100,000,000 divided by interface bandwidth in bits per second unless reference-bandwidth is reconfigured.
- ✗
1 Gbps
Why it's wrong here
1 Gbps is a frequently configured reference-bandwidth value on modern high-speed networks, not the IOS default. The default is 100 Mbps, giving cost 100,000,000 divided by bandwidth in bits per second; administrators raise the reference precisely because gigabit interfaces otherwise all yield cost 1.
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 and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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