Cisco IOS Debug Command Reference, Release 12.2
Using Debug Commands
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Using Debug Commands

Table Of Contents

Using Debug Commands

Entering debug Commands

Using the debug ? Command

Using the debug all Command

Generating debug Command Output

Redirecting debug and Error Message Output

Enabling Message Logging

Setting the Message Logging Levels

Limiting the Types of Logging Messages Sent to the Console

Logging Messages to an Internal Buffer

Limiting the Types of Logging Messages Sent to Another Monitor

Logging Messages to a UNIX Syslog Server

Limiting Messages to a Syslog Server


Using Debug Commands


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

Entering debug commands

Using the debug ? command

Using the debug all command

Generating debug command output

Redirecting debug and error message output


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 the command line in privileged EXEC mode at :

debug isdn q931

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

no debug isdn q931

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

undebug 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 Cisco engineers during the development process and are not intended for use outside the Cisco environment.

Using the debug all Command

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

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 performance of the router 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, the no debug modem command).

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

The router may not be properly configured to generate the type of traffic you want to monitor. Use the more system:running-config EXEC command to check its configuration.

Even if the router is properly configured, it may not generate the type of traffic you want to monitor during the particular period that debugging is turned on. Depending on the protocol you are debugging, you can use commands such as the TCP/IP ping EXEC command to generate network traffic.

Redirecting debug 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 debug output using a virtual terminal connection, rather than the console port.

To redirect debug 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 command:

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 devices:

Console

Monitor

Syslog server

Table 3 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 3 Message Logging Keywords and Levels 

Level
Keyword
Description
Syslog Definition

0

emergencies

System is unusable.

LOG_EMERG

1

alerts

Immediate action is needed.

LOG_ALERT

2

critical

Critical conditions exist.

LOG_CRIT

3

errors

Error conditions exist.

LOG_ERR

4

warnings

Warning conditions exist.

LOG_WARNING

5

notification

Normal, but significant, conditions exist.

LOG_NOTICE

6

informational

Informational messages.

LOG_INFO

7

debugging

Debugging messages.

LOG_DEBUG


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 logging keywords listed in Table 3. 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 show logging privileged EXEC command. 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 logging keywords listed in Table 3. To display logging messages on a terminal (virtual console), use the terminal monitor privileged EXEC command.

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 logging messages with a level up to and including the specified level argument. The level argument is one of the keywords listed in Table 3.

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:

Error messages about software or hardware malfunctions, displayed at the errors level.

Interface up/down transitions and system restart messages, displayed at the notification level.

Reload requests and low-process stack messages, displayed at the informational level.

Output from the debug commands, displayed at the debugging level.

The show logging privileged EXEC command 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 3 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