Which rule does an OSPF router follow for selecting a router ID when the OSPF process starts?

When an OSPF (Open Shortest Path First) router initiates the OSPF process, it follows a specific set of rules to select its router ID. The rule that holds the highest priority is the one that looks for an administratively configured router ID. If an OSPF router is configured with a router ID, it will use that specific value as its identification, affirming the priority of manually set configurations over automatically derived ones.

If there is no administratively assigned router ID present, the OSPF router will then seek out the highest IP address assigned to any of its active interfaces. This contingency ensures that the router has a unique identifier within the OSPF domain, which is crucial for maintaining routing integrity and communication among OSPF routers.

It's important to note that if a router ID is selected based on these rules, it cannot be changed while the OSPF process is running unless it is reconfigured and the OSPF process is restarted. This underscores the significance of properly configuring the router ID for OSPF, as it plays a critical role in OSPF operations and routing decisions.