Using an Async Modem on the AUX Port

Problem

You want to connect a standard asynchronous modem to the router’s AUX port and use it for dial backup.

Solution

Many Cisco routers include an AUX port that is a low-speed asynchronous serial interface that can connect to a standard modem and support PPP:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface Async65
Router2(config-if)#encapsulation ppp
Router2(config-if)#dialer in-band
Router2(config-if)#dialer pool-member 1
Router2(config-if)#ppp authentication chap
Router2(config-if)#async default routing
Router2(config-if)#exit
Router2(config)#interface Dialer1
Router2(config-if)#ip address 10.1.99.56 255.255.255.0
Router2(config-if)#encapsulation ppp
Router2(config-if)#dialer remote-name dialhost
Router2(config-if)#dialer pool 1
Router2(config-if)#dialer idle-timeout 300
Router2(config-if)#dialer string 95551212
Router2(config-if)#dialer-group 1
Router2(config-if)#ppp authentication chap
Router2(config-if)#exit
Router2(config)#line aux 0
Router2(config-line)#modem inout
Router2(config-line)#transport input all
Router2(config-line)#no exec
Router2(config-line)#speed 115200
Router2(config-line)#exit
Router2(config)#username dialhost password dialpassword
Router2(config)#ip route 0.0.0.0 0.0.0.0 10.1.99.1 180
Router2(config)#dialer-list 1 protocol ip list 101
Router2(config)#access-list 101 deny eigrp any any
Router2(config)#access-list 101 permit ip any any
Router2(config)#router ...

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