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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 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, 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:
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 Debug Commands
1
Using the Debug ? Command
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:
15:25:51:
15:25:51:
15:27:51:
15:27:51:
15:27:51:
15:27:52:
15:27:52:
15:27:57:
15:27:58:
TTY4:
tty4:
TTY4:
TTY4:
TTY4:
TTY4:
TTY4:
tty4:
TTY4:
TTY4:
DSR came up
Modem: IDLE->READY
Autoselect started
Autoselect failed
Line reset
Modem: READY->HANGUP
dropping DTR, hanging up
Modem: HANGUP->IDLE
restoring DTR
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:
•
•
The router may not be properly configured to generate the type of traffic you want to monitor.
Use the
more system:running-config
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.
2
Debug Command Reference
Redirecting Debugging and Error Message Output
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:
•
•
•
Console
Monitor
Syslog server
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.
Using Debug Commands
3
Redirecting Debugging and Error Message Output
Table 1
Level
0
1
2
3
4
5
6
7
Message Logging Keywords and Levels
Keyword
emergencies
alerts
critical
errors
warnings
notification
informational
debugging
Description
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.
Syslog Definition
LOG_EMERG
LOG_ALERT
LOG_CRIT
LOG_ERR
LOG_WARNING
LOG_NOTICE
LOG_INFO
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 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.
Example
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).
4
Debug Command Reference
Limiting the Types of Logging Messages Sent to Another Monitor
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.
Example
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:
•
•
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.
Using Debug Commands
5

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