Router ID

For OSPF to initialize, it must be able to define a router ID for the entire OSPF process. A router can receive its router ID from several sources. First, it can be assigned manually through the router-id command. Second, it is the numerically highest IP address set on a loopback interface. The loopback interface is a logical interface that never goes down. If no loopback address is defined, an OSPF enabled router will select the numerically highest IP address on any of its OSPF-configured interfaces as its router ID.

The router ID is chosen when OSPF is initialized. Initialization occurs when a router loads its OSPF configuration, whether at startup or when OSPF is first configured or reloaded. If other interfaces later come online that have a higher IP address, the OSPF router ID does not change until the OSPF process is restarted.