How is the OSPF interface cost calculated by default?

The OSPF (Open Shortest Path First) cost is calculated by default using the method of dividing the bandwidth reference by the interface's bandwidth. This calculation is central to how OSPF determines the best route for data transmission, as the cost represents the 'expense' associated with using a specific route.

The formula basically uses a fixed reference bandwidth, which is generally set to 100 Mbps in most implementations. So, if an interface has a bandwidth of 10 Mbps, the cost would be calculated by taking the reference bandwidth of 100 Mbps and dividing it by 10 Mbps, resulting in a cost of 10. This allows OSPF to dynamically adjust for different bandwidth capabilities and to emphasize the use of faster, more efficient links over slower ones in the routing process.

The other options do not accurately describe how OSPF calculates interface costs. User-defined values can be assigned to interfaces; however, the default mechanism relies explicitly on the bandwidth-based calculation. Doubling the interface bandwidth and assigning a constant value do not align with the way OSPF interprets link efficiency and its associated costs. Understanding this calculation method is crucial for configuring OSPF and optimizing network performance.