About this tool
OSPF chooses paths using interface cost. On many platforms the cost is derived from a reference bandwidth divided by interface bandwidth, though vendors allow manual cost configuration and may use different defaults.
The calculator helps you see what a given reference bandwidth produces on a link and why modern networks often raise the default reference value to distinguish 1G, 10G, 40G and 100G interfaces.
How to use it
- Enter the reference bandwidth used by your OSPF design.
- Enter the interface bandwidth.
- Calculate the resulting cost, then compare it with neighboring links and any manually configured costs.
Example: 100,000 Mbps reference with a 10,000 Mbps link
100,000 divided by 10,000 gives an OSPF cost of 10. A 100 Gbps link under the same reference would calculate to cost 1.
Engineering notes
- Keep reference-bandwidth settings consistent across the OSPF domain unless you have a deliberate design reason not to.
- Changing OSPF costs can move live traffic. Treat reference-bandwidth changes as routing changes and review their blast radius.
Frequently asked questions
Why is the old default reference bandwidth a problem?
A 100 Mbps reference can make Fast Ethernet and much faster interfaces all calculate to the minimum cost, eliminating useful path differentiation.
Can I set OSPF cost manually?
Yes on major routing platforms. A manual cost usually overrides the calculated interface cost.
Does lower OSPF cost win?
Yes. OSPF prefers the path with the lowest total accumulated cost to the destination.