Hello,
the AUX port on a Cisco router is seldom used, but it can be useful as an extra connection between two routers in a lab environment. For this example I used two Cisco 2516 router. The first step is to configure the AUX port.
line aux 0 modem InOut no exec transport input all speed 38400
The second step is two check which line number correspond to the AUX port.
r2516a#show line
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
* 0 CTY - - - - - 0 1 0/0 -
A 1 AUX 38400/38400 - inout - - - 1 0 0/0 -
2 VTY - - - - - 0 0 0/0 -
On this Cisco 2516 router line 1 is used for the AUX port. Third step, configure the async1 interface on the first router (called r2516a).
interface Async1 description from aux to aux r2516b ip address 192.168.255.249 255.255.255.252 encapsulation ppp async default routing async mode dedicated cdp enable end
Same AUX port configuration and same line number on the other router. The other router (called r2516b) has of course the other IP address of the /30 network that is used for the point to point connection.
interface Async1 description from aux to aux r2516a ip address 192.168.255.250 255.255.255.252 encapsulation ppp async default routing async mode dedicated cdp enable end
Fourth step, check the interfaces:
r2516a#sh ip int brief | i Async1 Async1 192.168.255.249 YES NVRAM up up r2516b#sh ip int brief | i Async1 Async1 192.168.255.250 YES NVRAM up up
Fifth and last step, ping the IP address of the other site:
r2516a#ping 192.168.255.250 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.255.250, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 76/80/88 ms r2516b#ping 192.168.255.249 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.255.249, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 76/80/84 ms
For this example I used a slow speed on the AUX port, you could try 57600 or even 115200. I used a short (only 20 cm) rollover cable 
to connect the two routers.
By the way, Cisco calls this method of connecting two routers via the AUX port a back-to-back connection.
Bye, Tore