A router ID is a 32-bit integer used to uniquely identify a device that uses a specific dynamic routing protocol for route exchange. A router ID uses the same format as an IP address. Router IDs are classified into global router IDs of devices and router IDs of dynamic routing protocols.
A router ID can be manually configured or automatically selected by a device. To ensure OSPF stability, you are advised to manually configure the IP address of a loopback interface as a router ID.
If the administrator does not configure a global router ID or configure a router ID for a specific dynamic routing protocol on the device, the router selects a router ID based on IP addresses of current interfaces:
If there are loopback interfaces that have IP addresses configured, the device selects the largest IP address among loopback interface addresses as the global router ID.
If no loopback interface is configured or loopback interfaces do not have IP addresses configured, the device selects the largest IP address among interface addresses as the global router ID without considering the Up/Down state of interfaces.
The device selects a router ID again only when the interface IP address that has been selected as the router ID is deleted or changed.
When a device is being initialized and there is no interface address on the device, the first configured IP address on the device, which may be a loopback interface address or another interface address, will become the global router ID of the device. This global router ID remains unchanged even if there is a larger interface IP address on the device. The device selects a new global router ID based on the preceding rules when the IP address that has been used as the existing global router ID is deleted.