What is the OSPF cost assigned to a loopback interface by default?

The OSPF (Open Shortest Path First) cost assigned to a loopback interface by default is 1. In OSPF, the cost is a metric used to determine the best path for data to travel through a network. A lower cost indicates a preferred route, and by convention, a loopback interface, which is a virtual interface that exists on a router, is assigned a default cost of 1. This default value enables OSPF to treat loopback interfaces as stable, reliable, and preferred paths, ensuring that they are always reachable. This behavior is important for proper routing and ensuring that routing protocols function optimally, as loopback interfaces often serve as stable points of reference for various network operations.

In contrast, the cost values assigned to other types of interfaces or paths can vary based on factors like bandwidth, which would not apply to loopbacks specifically. For example, Ethernet interfaces could have different costs based on the interface type, but loopback interfaces maintain a static cost.