Verifying and Troubleshooting the Automatic Configuration

Cisco provides many troubleshooting services to assist in the diagnostic process, and support
for troubleshooting the automatic configuration service is no exception. However, before
using the debug commands and other troubleshooting tools, it is best to review the status of
the installation and the connections between the modem and the router. Make sure to check
for the following:
 The modem is turned on and is receiving power.
 The cable is of the right type and is secured.
 The DIP switches or other physical options on the modem are set to known values or factory
defaults. In this case, known values means settings that are known to work for this
router and modem configuration in other installations; sometimes the factory defaults will
not work. In addition, administrators might find that random guessing is required to find
the proper settings.
 The modem is plugged into a phone jack and the dial tone is present.
After these steps have been completed, you need to reverse Telnet to communicate with the
modem. For reverse Telnet to work, the line interface needs the transport input all and modem
inout commands. These commands enable the port to accept input and transfer data to and from
the modem. Note that reverse Telnet is not a command but a tool used to provide a connection
to a reserved TCP port on the router, which maps to a physical asynchronous port. For example,
the physical port on line 4 would map to TCP port 2004. The administrator can telnet to the
router and, by altering the port number (the default TCP port for Telnet is 23), can be connected
directly to the attached device, such as a modem. TCP ports starting with 2000 are used for Telnet,
whereas 4000 is the start of the range for non-Telnet-specific TCP connections. Ports starting with
6000 are used for binary-mode Telnet. Of these, most administrators find it necessary to use ports
only in the 2000 range.
Reverse Telnet is a powerful tool that has been required for practical demonstrations
of Cisco expertise and certifications. Readers should be familiar with
its functionality.