Router-ID IP Addresses

In general, a device has IP addresses that are assigned to various interfaces. However, some routing protocols identify the device by the router ID, rather than by the IP addresses assigned to the interfaces connected by the protocol. If a router ID is not specified, such protocols use the following algorithm to select a router ID.

1. If the device has loopback interfaces, the router ID is the IP address on the loopback interface with the lowest IP address. For example, if you configure loopback interfaces 1, 2, and 3 as follows, the default router ID is 10.9.9.9/32:
   • Loopback 1: 10.9.9.9/32
   • Loopback 2: 10.4.4.4/32
   • Loopback 3: 10.1.1.1/32
2. If a loopback interface is not configured, then the lowest IP address configured on a physical interface becomes the router ID.