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Applying Cisco Troubleshooting Tools

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Nov 16, 2001.

Chapter Description

This sample chapter from CCNP Support Exam Certification Guide introduces some powerful troubleshooting tools that are built into the Cisco IOS. As with other tools, it is important that you identify when to use them and what information they reveal. Because some of these tools have an impact on the way routers operate and may impede the routers' utmost performance, it is essential to use them with care. To better understand the output of these commands, and to recognize what router internal operations they affect, this chapter discusses router internal components and operations. As each tool/command is introduced, its usefulness is described and tips are given on how to use it effectively.

Error Message Logging and Limiting the Display of Error Messages

Logging messages are important sources of information for network engineers in charge of troubleshooting. This section covers the following topics:

  • The options for the logging messages destination
  • Which destinations are the default for logging messages
  • Which commands enable/disable different destinations
  • How usage of different logging destinations compares with regard to the overhead they introduce on the routers
  • The eight levels of logging
  • Deciphering the logging messages

Message logging is enabled by default and it is directed to the console and the internal buffer. The privileged exec command logging on is the default setting, and if you enter no logging on, all logging except console logging will be turned off. The options for logging message destinations are:

  • Console
  • Internal buffer
  • Virtual terminal session (Telnet)
  • Syslog server (see Figure 4-5)

Figure 4-5 Error Message Logging Destinations

The following relationship demonstrates how different methods of logging compare in terms of the overhead they produce:

Buffered logging < Syslog < Virtual terminal < Console logging

Table 4-1 displays the commands you must use to configure the destination of logging messages and the desired level of logging for each destination.

Table 4-1 List of Commands for Logging Message Destinations


Usage Explanation

logging console [level]

This command turns console logging on and specifies the level of logging to be directed to the console. (The default setting is Enabled.)

The no logging console command disables console logging.

logging buffered [level]

Use this command to enable sending logging messages to the internal buffer (use no logging buffered to disable it) and specify the level of logging desired to be buffered. This feature is enabled by default.

logging monitor [level]

Use this command to enable sending logging messages to the virtual terminal sessions (use no logging monitor to disable it) and specify the level of logging desired to be directed to the virtual terminal lines.

From within a virtual terminal session, typing the command terminal monitor enables the display of logging messages. The command terminal no monitor turns this feature off.

logging trap [level]

This command allows you to enable sending logging messages to syslog servers and specify the level of these messages. The no logging trap command disables this feature.

The default level is Informational.

(Also see the explanation for the logging [ip-address] command below.)

logging [ip-address]

This command identifies the IP address of the syslog server so that the router can direct its logging messages to this address. Ifyou have a list of syslog servers to which you want to send the logging messages, you may enter this command with each server's appropriate address one by one. Use the no form of this command to take a server off the list.

There are eight levels of logging. If you specify a particular level of logging—for console logging, for example—the messages of that level and of the higher levels (numerically lower) are forwarded to the console. The levels of logging messages are explained in Table 4-2.

Table 4-2 Logging Messages


Logging Message

















Please note that when you enter one of the commands for specifying the level of logging to be directed to a particular place (console or virtual terminal sessions, for example), you must enter the English phrase for the level of logging and not the numeric value for it. For instance, the command to make virtual terminal lines receive logging messages at the errors level and higher (in other words errors, critical, alerts, emergencies) would be:

Router(config)# logging monitor error

Now let us discuss the anatomy of the logging messages. Each message is associated with one of the eight levels of logging, which is referred to as the severity of the message. Table 4-3 provides the eight levels of logging messages along with their associated severity levels and short descriptions. Logging messages are composed of a % sign followed by the facility, the severity, the mnemonic, and a text message. For instance, in this message:

%TR-3-WIREFAULT:Unit[0],wirefault:check the lobe cable MAU connection

the facility is Token Ring, severity is 3 (error), mnemonic is WIREFAULT, and, of course, the text message is reporting a wire fault condition.

Table 4-3 Logging Message Levels with Severity

Level Name



Syslog Definition



System unusable




Immediate action needed




Critical conditions




Error conditions




Warning conditions




Normal significant conditions




Informational messages only




Debugging messages


show logging Command

To display the state of syslog error and event logging, including host addresses, which type of logging (destination) is enabled, and other logging statistics, use the show logging (privileged EXEC) command. This command also displays the messages that are logged in the buffer.

Example 4-2 displays sample output of the show logging command.

Example 4-2 The show logging Command

Router#show logging

Syslog logging: enabled

Console logging: disabled

Monitor logging: level debugging, 13 messages logged.

Trap logging: level informational, 13 messages logged.

Logging to

SNMP logging: enabled, retransmission after 30 seconds

69 messages logged

Logging to, 0/10

Logging to, 0/10

Logging to, 0/10

7. Reachability and Step-by-Step Path Tests | Next Section Previous Section

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