How to Diagnose and Fix Serial-Line Speed Problems

Dial-in servers can experience problems because of conflicting speed settings. The following procedure helps you to isolate the cause of the link problem to conflicting serial-line speeds.

The following behaviors cause speed problems:

• You invoked PPP through a program such as /bin/login and specified the speed of the line.

• You started PPP from mgetty and accidentally supplied the bit rate.

pppd changes the speed that was originally set for the line to the speed that was set by /bin/login or mgetty. As a result, the line fails.

1. Log in to the dial-in server. Call the peer with debugging enabled.

   If you need instructions, see How to Turn on PPP Debugging.

2. Display the resulting /var/log/pppdebug log.

   Check the output for the following message:

   LCP too many configure requests

   This message indicates that the speeds of serial lines that were configured for PPP might potentially be in conflict.

3. Check if PPP is invoked through a program such as /bin/login and the line speed that was set.

   In such a situation, pppd changes the originally configured line speed to the speed that is specified in /bin/login.

4. Check if a user started PPP from the mgetty command and accidentally specified a bit rate.

   This action also causes serial-line speeds to conflict.

5. Fix the conflicting serial-line speed problem as follows:
   1. Lock the DTE rate on the modem.
   2. Do not use autobaud.
   3. Do not change the line speed after configuration.