Changing the reference bandwidth for the cost on OSPFv2 interfaces

Each interface on which OSPFv2 is enabled has a cost associated with it. The device advertises its interfaces and their costs to OSPFv2 neighbors. For example, if an interface has an OSPFv2 cost of ten, the device advertises the interface with a cost of ten to other OSPFv2 routers.

By default, an interface’s OSPFv2 cost is based on the port speed of the interface. The cost is calculated by dividing the reference bandwidth by the port speed. The default reference bandwidth is 100 Mbps, which results in the following default costs:

• 10 Mbps port - 10
• All other port speeds - 1

You can change the reference bandwidth. The following formula is used to calculate the cost:

Cost = reference-bandwidth/interface-speed

If the resulting cost is less than 1, the cost is rounded up to 1. The default reference bandwidth results in the following costs:

• 10 Mbps port’s cost = 100/10 = 10
• 100 Mbps port’s cost = 100/100 = 1
• 1000 Mbps port’s cost = 100/1000 = 0.10, which is rounded up to 1
• 10 Gbps port’s cost = 100/10000 = 0.01, which is rounded up to 1

The bandwidth for interfaces that consist of more than one physical port is calculated as follows:

• LAG group - The combined bandwidth of all the ports.
• Virtual interface - The combined bandwidth of all the ports in the port-based VLAN that contains the virtual interface.

The default reference bandwidth is 100 Mbps. You can change the reference bandwidth to a value from 1—4294967.

If a change to the reference bandwidth results in a cost change to an interface, the device sends a link-state update to update the costs of interfaces advertised by the device.