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Using Debug Commands

Using Debug Commands

This chapter explains how you use debug commands to diagnose and resolve internetworking problems. Specifically, it covers the following topics:

Caution Because debugging output is assigned high priority in the CPU process, it can render the system unusable. For this reason, use debug commands only to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. Moreover, it is best to use debug commands during periods of lower network traffic and fewer users. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system use.

Entering Debug Commands

All debug commands are entered in privileged EXEC mode, and most debug commands take no arguments. For example, to enable the debug isdn q931 command, enter the following in privileged EXEC mode at the command line:

debug isdn q931

To turn off the debug isdn q931 command, in privileged EXEC mode, enter the no form of the command at the command line:

no debug isdn q931

Alternately, in privileged EXEC mode, you can enter the undebug form of the command:

debug isdn q931

To display the state of each debugging option, enter the following at the command line in privileged EXEC mode:

show debugging

Using the Debug ? Command

To list and see a brief description of all the debugging command options, enter the following command in privileged EXEC mode at the command line:

debug  ?

Not all debugging commands listed in the debug  ? output are described in this document. Commands are included here based on the their usefulness in assisting you to diagnose network problems. Commands not included are typically used internally by engineering during the development process and are not intended for use outside this environment.

Using the Debug All Command

To enable all system diagnostics, enter the following command in privileged EXEC mode at the command line:

debug all

The no debug all command turns off all diagnostic output. Using the no debug all command is a convenient way to ensure that you have not accidentally left any debug commands turned on.

Caution Because debugging output takes priority over other network traffic, and because the debug all command generates more output than any other debug command, it can severely diminish the router's performance or even render it unusable. In virtually all cases, it is best to use more specific debug commands.

Generating Debug Command Output

Enabling a debug command can result in output similar to the following example for the debug modem command.

Router# debug modem
15:25:51: TTY4: DSR came up
15:25:51: tty4: Modem: IDLE->READY
15:25:51: TTY4: Autoselect started
15:27:51: TTY4: Autoselect failed
15:27:51: TTY4: Line reset
15:27:51: TTY4: Modem: READY->HANGUP
15:27:52: TTY4: dropping DTR, hanging up
15:27:52: tty4: Modem: HANGUP->IDLE
15:27:57: TTY4: restoring DTR
15:27:58: TTY4: DSR came up

The router continues to generate such output until you enter the corresponding no debug command (in this case, no debug modem).

If you enable a debug command and no output is displayed, consider the following possibilities:

Redirecting Debugging and Error Message Output

By default, the network server sends the output from debug commands and system error messages to the console. If you use this default, monitor debugging output using a virtual terminal connection, rather than the console port.

To redirect debugging output, use the logging command options within configuration mode as described in the following sections.

Possible destinations include the console, virtual terminals, internal buffer, and UNIX hosts running a syslog server. The syslog format is compatible with 4.3 Berkeley Standard Distribution (BSD) UNIX and its derivatives.

Note Be aware that the debugging destination you use affects system overhead. Logging to the console produces very high overhead, whereas logging to a virtual terminal produces less overhead. Logging to a syslog server produces even less, and logging to an internal buffer produces the least overhead of any method.

To configure message logging, you need to be in configuration command mode. To enter this mode, use the configure terminal command at the EXEC prompt.

Enabling Message Logging

To enable message logging to all supported destinations other than the console, enter the following:

logging on

The default condition is logging on.

To direct logging to the console only and disable logging output to other destinations, enter the following command:

no logging on

Setting the Message Logging Levels

You can set the logging levels when logging messages to the following:

Table 1 lists and briefly describes the logging levels and corresponding keywords you can use to set the logging levels for these types of messages. The highest level of message is level 0, emergencies. The lowest level is level 7, debugging, which also displays the greatest amount of messages. For information about limiting these messages, see sections later in this chapter.

Table 1: Message Logging Keywords and Levels
Level Keyword Description Syslog Definition



System is unusable.




Immediate action is needed.




Critical conditions exist.




Error conditions exist.




Warning conditions exist.




Normal, but significant, conditions exist.




Informational messages.




Debugging messages.


Limiting the Types of Logging Messages Sent to the Console

To limit the types of messages that are logged to the console, use the logging console router configuration command. The full syntax of this command follows:

logging console level
no logging console

The logging console command limits the logging messages displayed on the console to messages up to and including the specified severity level, which is specified by the level argument.

The level argument is one of the keywords listed in Table 1. Keywords are listed in order from the most severe level to the least severe.

The no logging console command disables logging to the console.


The following example sets console logging of messages at the debugging level, which is the least severe level and which displays all logging messages:

logging console debugging

Logging Messages to an Internal Buffer

The default logging device is the console; all messages are displayed on the console unless otherwise specified.

To log messages to an internal buffer, use the logging buffered router configuration command. The full syntax of this command follows:

logging buffered
no logging buffered

The logging buffered command copies logging messages to an internal buffer instead of writing them to the console. The buffer is circular in nature, so newer messages overwrite older messages. To display the messages that are logged in the buffer, use the privileged EXEC command show logging. The first message displayed is the oldest message in the buffer.

The no logging buffered command cancels the use of the buffer and writes messages to the console (the default).

Limiting the Types of Logging Messages Sent to Another Monitor

To limit the level of messages logged to the terminal lines (monitors), use the logging monitor router configuration command. The full syntax of this command follows:

logging monitor level
no logging monitor

The logging monitor command limits the logging messages displayed on terminal lines other than the console line to messages with a level up to and including the specified level argument. The level argument is one of the keywords listed in Table 1. To display logging messages on a terminal (virtual console), use the privileged EXEC command terminal monitor.

The no logging monitor command disables logging to terminal lines other than the console line.


The following example sets the level of messages displayed on monitors other than the console to notification:

logging monitor notification

Logging Messages to a UNIX Syslog Server

To log messages to the syslog server host, use the logging router configuration command. The full syntax of this command follows:

logging ip-address
no logging ip-address

The logging command identifies a syslog server host to receive logging messages. The ip-address argument is the IP address of the host. By issuing this command more than once, you build a list of syslog servers that receive logging messages.

The no logging command deletes the syslog server with the specified address from the list of syslogs.

Limiting Messages to a Syslog Server

To limit the number of messages sent to the syslog servers, use the logging trap router configuration command. The full syntax of this command follows:

logging trap level
no logging trap

The logging trap command limits the logging messages sent to syslog servers to messages with a level up to and including the specified level argument. The level argument is one of the keywords listed in Table 1.

To send logging messages to a syslog server, specify its host address with the logging command.

The default trap level is informational.

The no logging trap command disables logging to syslog servers.

The current software generates four categories of syslog messages:

The privileged EXEC command show logging displays the addresses and levels associated with the current logging setup. The command output also includes ancillary statistics.

Example of Setting Up a UNIX Syslog Daemon

To set up the syslog daemon on a 4.3 BSD UNIX system, include a line such as the following in the file /etc/syslog.conf:

local7.debugging /usr/adm/logs/tiplog

The local7 keyword specifies the logging facility to be used.

The debugging keyword specifies the syslog level. See Table 1 for other keywords that can be listed.

The UNIX system sends messages at or above this level to the specified file, in this case /usr/adm/logs/tiplog. The file must already exist, and the syslog daemon must have permission to write to it.

For the System V UNIX systems, the line should read as follows:

local7.debug /usr/admin/logs/cisco.log



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