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Understanding Cisco System Message Logging

This post by David Papkin is about Configuring Cisco System Message Logging

By default, a switch sends the output from system messages and debug privileged EXEC commands to a logging process. The logging process controls the distribution of logging messages to various destinations, such as the logging buffer, terminal lines, or a UNIX syslog server, depending on your configuration. The process also sends messages to the console.

System Log Message Format

System log messages can contain up to 80 characters and a percent sign (%), which follows the optional sequence number or time-stamp information, if configured. Messages appear in this format:

seq no:timestamp: %facility-severity-MNEMONIC:description

The part of the message preceding the percent sign depends on the setting of the service sequence-numbers, service timestamps log datetime, service timestamps log datetime [localtime] [msec] [show-timezone], orservice timestamps log uptime global configuration command.

Table 27-1 describes the elements of syslog messages.

Table 27-1 System Log Message Elements

seq no:

Stamps log messages with a sequence number only if the service sequence-numbers global configuration command is configured.

For more information, see the “Enabling and Disabling Sequence Numbers in Log Messages” section.

timestamp formats:

mm/dd hh:mm:ss

or

hh:mm:ss (short uptime)

or

d h (long uptime)

Date and time of the message or event. This information appears only if the service timestamps log [datetime | log] global configuration command is configured.

For more information, see the “Enabling and Disabling Time Stamps on Log Messages” section.

facility

The facility to which the message refers (for example, SNMP, SYS, and so forth). For a list of supported facilities, see Table 27-4.

severity

Single-digit code from 0 to 7 that is the severity of the message. For a description of the severity levels, see Table 27-3.

MNEMONIC

Text string that uniquely describes the message.

description

Text string containing detailed information about the event being reported.

This example shows a partial switch system message:

00:00:46: %LINK-3-UPDOWN: Interface Port-channel1, changed state to up

00:00:47: %LINK-3-UPDOWN: Interface GigabitEthernet0/1, changed state to up

00:00:47: %LINK-3-UPDOWN: Interface GigabitEthernet0/2, changed state to up

00:00:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down

00:00:48: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/1, changed 
state to down 2

*Mar  1 18:46:11: %SYS-5-CONFIG_I: Configured from console by vty2 (10.34.195.36)

18:47:02: %SYS-5-CONFIG_I: Configured from console by vty2 (10.34.195.36)

*Mar  1 18:48:50.483 UTC: %SYS-5-CONFIG_I: Configured from console by vty2 (10.34.195.36) 

Default System Message Logging Configuration

Table 27-2 shows the default system message logging configuration.

Disabling and Enabling Message Logging

Message logging is enabled by default. It must be enabled to send messages to any destination other than the console. When enabled, log messages are sent to a logging process, which logs messages to designated locations asynchronously to the processes that generated the messages.

Beginning in privileged EXEC mode, follow these steps to disable message logging:

Disabling the logging process can slow down the switch because a process must wait until the messages are written to the console before continuing. When the logging process is disabled, messages appear on the console as soon as they are produced, often appearing in the middle of command output.

The logging synchronous global configuration command also affects the display of messages to the console. When this command is enabled, messages appear only after you press Return. For more information, see the“Synchronizing Log Messages” section.

To re-enable message logging after it has been disabled, use the logging on global configuration command.

Setting the Message Display Destination Device

If message logging is enabled, you can send messages to specific locations in addition to the console. Beginning in privileged EXEC mode, use one or more of the following commands to specify the locations that receive messages:

The logging buffered global configuration command copies logging messages to an internal buffer. The buffer is circular, so newer messages overwrite older messages after the buffer is full. 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. To clear the contents of the buffer, use the clear logging privileged EXEC command.

To disable logging to the console, use the no logging console global configuration command. To disable logging to a file, use the no logging file [severity-level-number | type] global configuration command.

Synchronizing Log Messages

You can configure the system to synchronize unsolicited messages and debug privileged EXEC command output with solicited device output and prompts for a specific console port line or virtual terminal line. You can identify the types of messages to be output asynchronously based on the level of severity. You can also determine the maximum number of buffers for storing asynchronous messages for the terminal after which messages are dropped.

When synchronous logging of unsolicited messages and debug command output is enabled, unsolicited device output appears on the console or is printed after solicited device output appears or is printed. Unsolicited messages and debug command output appears on the console after the prompt for user input is returned. Therefore, unsolicited messages and debug command output are not interspersed with solicited device output and prompts. After the unsolicited messages appear, the console again displays the user prompt.

Beginning in privileged EXEC mode, follow these steps to configure synchronous logging:

To disable synchronization of unsolicited messages and debug output, use the no logging synchronous [level severity-level | all] [limit number-of-buffers] line configuration command.

Enabling and Disabling Time Stamps on Log Messages

By default, log messages are not time-stamped.

Beginning in privileged EXEC mode, follow these steps to enable time-stamping of log messages:

To disable time stamps for both debug and log messages, use the no service timestamps global configuration command.

This example shows part of a logging display with the service timestamps log datetime global configuration command enabled:

*Mar  1 18:46:11: %SYS-5-CONFIG_I: Configured from console by vty2 (10.34.195.36)

This example shows part of a logging display with the service timestamps log uptime global configuration command enabled:

00:00:46: %LINK-3-UPDOWN: Interface Port-channel1, changed state to up

Enabling and Disabling Sequence Numbers in Log Messages

Because there is a chance that more than one log message can have the same time stamp, you can display messages with sequence numbers so that you can unambiguously refer to a single message. By default, sequence numbers in log messages are not displayed.

Beginning in privileged EXEC mode, follow these steps to enable sequence numbers in log messages:

To disable sequence numbers, use the no service sequence-numbers global configuration command.

This example shows part of a logging display with sequence numbers enabled:

000019: %SYS-5-CONFIG_I: Configured from console by vty2 (10.34.195.36)

Defining the Message Severity Level

You can limit messages displayed to the selected device by specifying the severity level of the message, which are described in Table 27-3.

Beginning in privileged EXEC mode, follow these steps to define the message severity level:

Note Specifying a level causes messages at that level and numerically lower levels to be displayed at the destination.

To disable logging to the console, use the no logging console global configuration command. To disable logging to a terminal other than the console, use the no logging monitor global configuration command. To disable logging to syslog servers, use the no logging trap global configuration command.

Table 27-3 describes the level keywords. It also lists the corresponding UNIX syslog definitions from the most severe level to the least severe level.

The software generates four other categories of messages:

•Error messages about software or hardware malfunctions, displayed at levels warnings through emergencies. These types of messages mean that the functionality of the switch is affected. For information on how to recover from these malfunctions, refer to the system message guide for this release.

•Output from the debug commands, displayed at the debugging level. Debug commands are typically used only by the Technical Assistance Center.

•Interface up or down transitions and system restart messages, displayed at the notifications level. This message is only for information; switch functionality is not affected.

•Reload requests and low-process stack messages, displayed at the informational level. This message is only for information; switch functionality is not affected.

Limiting Syslog Messages Sent to the History Table and to SNMP

If you enabled syslog message traps to be sent to an SNMP network management station by using the snmp-server enable trap global configuration command, you can change the level of messages sent and stored in the switch history table. You also can change the number of messages that are stored in the history table.

Messages are stored in the history table because SNMP traps are not guaranteed to reach their destination. By default, one message of the level warning and numerically lower levels (see Table 27-3) are stored in the history table even if syslog traps are not enabled.

Beginning in privileged EXEC mode, follow these steps to change the level and history table size defaults:

When the history table is full (it contains the maximum number of message entries specified with the logging history size global configuration command), the oldest message entry is deleted from the table to allow the new message entry to be stored.

To return the logging of syslog messages to the default level, use the no logging history global configuration command. To return the number of messages in the history table to the default value, use the no logging history sizeglobal configuration command.

Configuring UNIX Syslog Servers

The next sections describe how to configure the UNIX server syslog daemon and how to define the UNIX system logging facility.

Logging Messages to a UNIX Syslog Daemon

Before you can send system log messages to a UNIX syslog server, you must configure the syslog daemon on a UNIX server. Log in as root, and perform these steps:

Note Some recent versions of UNIX syslog daemons no longer accept by default syslog packets from the network. If this is the case with your system, use the UNIX man syslogd command to determine what options must be added to or removed from the syslog command line to enable logging of remote syslog messages.

Step 1 Add a line such as the following to the file /etc/syslog.conf:

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

The local7 keyword specifies the logging facility to be used; see Table 27-4 for information on the facilities. The debug keyword specifies the syslog level; see Table 27-3 for information on the severity levels. The syslog daemon sends messages at this level or at a more severe 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.

Step 2 Create the log file by entering these commands at the UNIX shell prompt:

$ touch /var/log/cisco.log

$ chmod 666 /var/log/cisco.log

Step 3 Make sure the syslog daemon reads the new changes:

$ kill -HUP `cat /etc/syslog.pid`

For more information, see the man syslog.conf and man syslogd commands on your UNIX system.

Configuring the UNIX System Logging Facility

When sending system log messages to an external device, you can cause the switch to identify its messages as originating from any of the UNIX syslog facilities.

Beginning in privileged EXEC mode, follow these steps to configure UNIX system facility message logging:

To remove a syslog server, use the no logging host global configuration command, and specify the syslog server IP address. To disable logging to syslog servers, enter the no logging trap global configuration command.

Table 27-4 lists the UNIX system facilities supported by the Cisco IOS software. For more information about these facilities, consult the operator’s manual for your UNIX operating system.

Displaying the Logging Configuration

To display the logging configuration and the contents of the log buffer, use the show logging privileged EXEC command. For information about the fields in this display, refer to the Cisco IOS Configuration Fundamentals Command Reference for Cisco IOS Release 12.1.

Note The syslog format is compatible with 4.3 BSD UNIX.

When the logging process is disabled, messages are sent only to the console. The messages are sent as they are generated, so message and debug output are interspersed with prompts or output from other commands. Messages appear on the console after the process that generated them has finished.

You can set the severity level of the messages to control the type of messages displayed on the console and each of the destinations. You can time-stamp log messages or set the syslog source address to enhance real-time debugging and management. For information on possible messages, refer to the system message guide for this release.

You can access logged system messages by using the switch command-line interface (CLI) or by saving them to a properly configured syslog server. The switch software saves syslog messages in an internal buffer. You can remotely monitor system messages by accessing the switch through Telnet, through the console port, or by viewing the logs on a syslog server.

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2950/software/release/12-1_19_ea1/configuration/guide/2950scg/swlog.html

This concludes this post by David Papkin about Configuring message logging in Cisco