You will, at some point, need to troubleshoot your modem and its configuration. Debug output is usually crucial to this process. As always with debugging, only turn on that which is absolutely necessary and turn it off as soon as you are done. Debug commands can produce substantial amounts of output and can have a rather dramatic (and negative) effect on the functionality of your router. Debugging Modem AutoconfigurationIf you suspect that your modem is not being detected during autoconfiguration or discovery, debug is the first thing to check. From an enable prompt, issue the debug confmodem command. You see each string that the router is trying and the current bit rate. If after all the initialization strings are tried at all bit rates, you have not detected your modem, there is something wrong at the physical level: Router#debug confmodem Modem Configuration Database debugging is on Jan 1 01:01:01.000: TTY1: detection speed (115200) response ---OK--- Jan 1 01:01:01.000: TTY1: Modem command: --AT&FS0=1&D3-- Jan 1 01:01:02.000: TTY1: Modem configuration succeeded Jan 1 01:01:02.000: TTY1: Detected modem speed 115200 Jan 1 01:01:02.000: TTY1: Done with modem configuration In addition to issuing the debug command, it is advisable to use show line to see which lines are currently in use (and have been configured) and use clear line if necessary. Occasionally, a modem encounters a problem and does not reset when it loses the carrier signal. If this problem happens, the router doesn't actually know to redetect the modem and a clear line resets the process: Router#show line Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns * 0 CTY - - - - - 0 0 0/0 * 1 TTY 115200/115200 - inout - - - 0 0 0/0 65 AUX 9600/9600 - - - - - 0 0 0/0 * 66 VTY - - - - - 8 0 0/0 67 VTY - - - - - 3 0 0/0 68 VTY - - - - - 1 0 0/0 69 VTY - - - - - 0 0 0/0 70 VTY - - - - - 0 0 0/0 Router#clear line 1 The * in the show line output shows that a line is in use, and the clear line command uses the line number from the show line output.
Additional TroubleshootingOne common mistake, or overlooked step, is to make sure that the serial interface is configured for asynchronous mode. If you cannot get into asynchronous interface mode, you have probably forgotten this step: Router(config-if)#physical-layer async Once you configure the serial interface to be asynchronous, it can communicate with the modem. If debugging shows that the modem isn't being detected, check the following:
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