CISCO IOS in a Nutshell

The Auxiliary (AUX) Port

The router’s auxiliary (AUX) port functions as a backup async port. It is most commonly used as a backup console port, but it can also be used as a dial-up port for remote router management and many other functions. It doesn’t have the performance of an asynchronous line; its speed is often limited (particularly on older routers), and it does only per-character I/O, which creates a high CPU load if used continuously.

Figure 4-4 shows how you might use the AUX port as a backup for a T1 line. If the T1 connection goes down, Router 1 automatically dials Router 2 using the modem connected to the AUX port. Obviously, the speed of the backup link is not comparable to the T1 connection, but it does provide some level of backup support.

Figure 4-4. Using an AUX port as a backup connection

The following configuration examples show how to use the AUX port as backup connection. This example uses many commands that are well beyond the scope of this chapter; they are included here to make the configuration complete. The comments describe some of the more advanced commands; for more information about PPP and dial-on-demand routing, see Chapter 11.

!  Configure the primary (T1) interface, with async 4 as a backup. 
!  See Chapter 5 for info on interface configuration.
!  See Chapter 11 for the dialing commands interface serial0 ip address 10.10.1.1 255.255.255.0 backup interface ...