System information

Establishing a Reverse Telnet Session to a Modem
Book Title
16-344
Use the following procedure to configure a Cisco router to automatically attempt to discover what
kind of modem is connected to the line and then to configure the modem:
Step 1 To discover the type of modem attached to your router, use the modem autoconfigure
discovery line configuration command.
Step 2 When the modem is successfully discovered, configure the modem automatically using
the modem autoconfigure type modem-name line configuration command.
If you want to display the list of modems for which the router has entries, use the show modemcap
modem-name. If you want to change a modem value that was returned from the show modemcap
command, use the modemcap edit modem-name attribute value line configuration command.
For complete information on the use of these commands, refer to the Cisco IOS Access Services
Configuration Guide and Access Services Command Reference.
Establishing a Reverse Telnet Session to a Modem
If you are running Cisco IOS Release 11.0 or earlier, you must establish a reverse Telnet session to
configure a modem to communicate with a Cisco device. As long as you lock the data terminal
equipment (DTE)–side speed of the modem (see Table 16-5 for information on locking the modem
speed), the modem will always communicate with the access server or router at the desired speed.
Be certain that the speed of the Cisco device is configured prior to issuing commands to the modem
via a reverse Telnet session. (See Table 16-5 for information on configuring the speed of the access
server or router.)
To initiate a reverse Telnet session to your modem, perform the following steps:
Step 1 From your terminal, use the command
telnet ip-address 20yy
where ip-address is the IP address of any active, connected interface on the Cisco device,
and yy is the line number to which the modem is connected. For example, the following
command would connect you to the auxiliary port on a Cisco router with IP address
192.169.53.52:
telnet 192.169.53.52 2001
Generally, a Telnet command of this kind can be issued from anywhere on the network
that can ping the IP address in question.
Note On a Cisco router, port 01 is the auxiliary port. On a Cisco access server, the auxiliary port is
another last_tty+1, so on a 16-port access server, the auxiliary port is port 17. Use the show line exec
command to make certain you are working with the correct line.
Step 2 If the connection is refused, there may already be a user connected to that port. Use the
show users exec command to determine whether the line is being used. If desired, the line
can be cleared from the console using the clear line privileged exec command. When you
are certain the line is not in use, attempt the Telnet connection again.
Step 3 If the connection is again refused, confirm that you have set modem control to
modem inout for that line. See Table 16-2 for more information on configuring modem
control on a line.
Step 4 If the connection is still refused, the modem might be asserting Carrier Detect (CD) all
the time. Disconnect the modem from the line, establish a reverse Telnet session, and then
connect the modem.