User guide

Internetworking Primer A-43

Modems

Communicating with the Modem

You must establish communication with your modem before you can conﬁgure it, which

requires terminal access to the modem’s command environment. The access server’s

reverse Telnet feature is used to communicate with the modem. This section explains how

to use reverse Telnet to access the modem.

Initiating a Reverse Telnet

To initiate a reverse Telnet, determine the IP address of your LAN (Ethernet) interface, then

enter a Telnet command to port 2000 + n on the access server, where n is the line number

for the modem to be conﬁgured. For example, to connect to the modem on line 1, enter the

following command from an EXEC session on the access server:

Router# telnet 172.16.1.10 2001

Trying 172.16.1.10, 2001 ... Open

You can now communicate with the modem on line 1 using the at command set as deﬁned

by the modem vendor.

Timesaver Use the conﬁguration command ip host to simplify reverse Telnets.

For example, ip host modem1 2001 172.16.1.10.

If you are unable to connect to the modem, check the following:

1 The EXEC command show users should not indicate the line is in use.

2 The line should be conﬁgured for modem inout.

3 The output of the EXEC command show line value should contain the following two

lines:

Modem state: Idle

Modem hardware state: CTS noDSR DTR RTS

4 The line virtual terminal connections in the access server conﬁguration may require

passwords. See the publication Troubleshooting Internetworks for additional

information on assigning passwords to virtual terminals.