OSPF reference bandwidth is the baseline used to calculate link cost: reference bandwidth divided by link bandwidth. The default is 100 Mbps, and fractional results become a cost of 1. section 18.1.3 section 18.5
With the default 100-Mbps reference bandwidth:
- A 10-Mbps link has cost 10; a 100-Mbps link has cost 1. section 18.1.3
- A 1-Gbps or 10-Gbps link also has cost 1, making unlike high-speed links appear equally desirable. section 18.1.3
- A higher reference bandwidth preserves meaningful cost differences and helps avoid unsuitable ECMP across unequal links. section 18.1.3
Changing the reference bandwidth
In OSPF router configuration mode, enter `auto-cost reference-bandwidth 1000` to set the reference to 1,000 Mbps. A GigabitEthernet interface remains cost 1, while a FastEthernet interface changes from cost 1 to cost 10. Keep the reference bandwidth consistent across all routers.
