Most Cisco routers include an additional auxiliary (Aux) port as a backup async port. This port is commonly used as a dial-up port for remote router management. It is connected to a modem and enables an administrator to make a phone call to connect to the router’s CLI.
The following figure shows how to connect a modem to an aux port on a router (image source: Cisco):
The network administrator uses a terminal emulation program to connect to the router over the aux port. The configuration of the aux port resembles the the console port configuration:
Router(config)#line aux 0 Router(config-line)#password cisco Router(config-line)#login
The code above enables password checking for users dialing into the router.