Using Debug Commands

This chapter explains how you can 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 debugging output

• Redirecting debugging output

Caution Because debugging output is assigned high priority in the CPU process, it can render the system unusable. For this reason, only use debug commands 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 reduces the effect these commands have on other users on the system.

Entering Debug Commands

All debug commands are entered while in privileged EXEC mode and most debug commands do not take any arguments. For example, to enable the debug broadcast command, enter the following in privileged EXEC mode at the command line:

debug broadcast

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

no debug broadcast

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

undebug broadcast

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 briefly describe all of the debugging command options, enter the following command in privileged mode at the command line:

debug ?

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 example shown in Figure 1-1 for the debug broadcast command.

Figure 1-1: Example Debug Broadcast Output

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

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 write terminal 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 command to generate network traffic.

Redirecting Debugging and Error Message Output

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

To redirect debugging output, use the logging command options within configuration mode.

Possible destinations include the console terminal, virtual terminals, and UNIX hosts running a syslog server. The syslog format is compatible with 4.3 BSD UNIX and its derivatives.

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

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

The following sections describe how to implement the redirection options.

Enabling Message Logging

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

logging on

This behavior is the default.

To direct logging to the console terminal only, enter the following:

no logging on

Setting the Message Logging Levels

You can set the logging levels when logging the following:

• Messages sent to console

• Messages sent to another monitor

• Messages sent to a syslog server

Table 1-1 lists and briefly describes the logging levels and corresponding keywords that you can use to set the logging levels for these types of messages. For information about limiting these messages, see sections later in this chapter.

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 terminal to messages with a level number at or below the specified severity level, which is specified by the level argument.

The level argument can be one of the keywords listed in Table 1-1. They are listed in order from the most severe level to the least severe.

The no logging console command disables logging to the console terminal.

Example

This command sets console logging of messages at the debugging level:

! 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 terminal. The buffer is circular in nature, so newer messages overwrite older messages. To display the messages that are logged in the buffer, use the 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 terminal (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 at or above the value of the level variable. The level argument is one of the keywords described for the logging console command in Table 1-1. 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.

Example

This command 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 internet-address
no logging internet-address

The logging command identifies a syslog server host to receive logging messages. The internet-address argument is the Internet 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 how many messages are sent to the syslog servers, use the logging trap router configuration command. Its full syntax follows:

logging trap level
no logging trap

The logging trap command limits the logging messages sent to syslog servers to messages with a level at or above the value of the level variable. The level argument is one of the keywords described for the logging console command in Table 1-1.

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:

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

Example

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.debug /usr/adm/logs/tiplog

The local7 keyword specifies the logging facility to be used.

The debug argument specifies the syslog level. See Table 1-1 for other arguments that can be listed.

The UNIX system sends messages at or below this level to the file specified in the next field. The file must already exist, and the syslog daemon must have permission to write to it.