OSPF calculates interface cost using this formula:
Cost = Reference Bandwidth / Interface Bandwidth
The default reference bandwidth is Mbps ( bps). Examples:
- Mbps Ethernet:
- Mbps Fast Ethernet:
- Gbps: , rounded to
Problem: FastEthernet, GigE, and GigE all get cost . OSPF can't distinguish between them.
Solution: Increase reference bandwidth. With Mbps:
- Mbps: cost
- Gbps: cost
- Gbps: cost
Set this consistently across all routers.