Automatic router ID selection

The first detected IPv4 address becomes the router ID. Prior to a reboot, this can be any IPv4 address configured on the routing switch. Following a reboot, the router ID is set to the lowest IPv4 loopback address detected.

If multiple IPv4 loopback addresses are detected at reboot, the address configured on the lowest-numbered IPv4 loopback interface (lo0 through lo7) becomes the router ID. If the lowest-numbered loopback interface has multiple IPv4 addresses, the lowest of these addresses is selected as the router ID. Once a router ID is selected, it does not automatically change unless a higher-priority address is configured on the routing switch and the routing protocol is restarted with a reboot.