What does an OSPF router prefer to use first as a router ID?

The preferred method for an OSPF router to determine its router ID is by using any IP address configured with the router-id command. This command allows network administrators to explicitly set a router's OSPF router ID, which is beneficial for ensuring consistency and stability in routing decisions across the OSPF network.

When a router ID is set manually using the router-id command, OSPF uses this defined value regardless of the router's interface IP addresses. This takes precedence over any automatically assigned identifiers, giving administrators control over what OSPF identifies as the router's ID. Proper configuration of the router ID is crucial in multi-access networks, helping to avoid potential issues associated with default selection processes.

While the highest IP address of all interfaces and the address of the loopback interface are considered in determining the router ID, they come into play only if no router ID has been set explicitly. The loopback interface is often favored in scenarios where a static identification method is desired, but it's not the first choice when a manual ID exists. As such, using the router-id command guarantees the desired identification for OSPF routing functions.