System information

CHAPTER
Troubleshooting Serial Line Problems 15-313
15
Troubleshooting Serial Line Problems
This chapter presents general troubleshooting information and a discussion of tools and techniques
for troubleshooting serial connections. The chapter consists of the following sections:
• Troubleshooting Using the show interfaces serial Command
• Using the show controllers Command
• Using debug Commands
• Using Extended ping Tests
• Troubleshooting Clocking Problems
• Adjusting Buffers
• Special Serial Line Tests
• Detailed Information on the show interfaces serial Command
Troubleshooting Using the show interfaces serial Command
The output of the show interfaces serial exec command displays information specific to serial
interfaces. Figure 15-1 shows the output of the show interfaces serial exec command for a
High-Level Data Link Control (HDLC) serial interface.
This section describes how to use the show interfaces serial command to diagnose serial line
connectivity problems in a wide-area network (WAN) environment. The following sections describe
some of the important fields of the command output.
Other fields shown in the display are described in detail in the section “Detailed Information on the
show interfaces serial Command” later in this chapter.
Serial Lines: show interfaces serial Status Line Conditions
You can identify five possible problem states in the interface status line of the show interfaces serial
display (see Figure 15-1):
• Serial x is down, line protocol is down
• Serial x is up, line protocol is down
• Serial x is up, line protocol is up (looped)
• Serial x is up, line protocol is down (disabled)
• Serial x is administratively down, line protocol is down