Command:

logging synchronous


Mode:

Router(config-line)#


Syntax:

logging synchronous [level {severity-level | all}] [limit number-of-buffers]
no logging synchronous

 

Syntax Description:

level severity-level | all (Optional) Specifies the message severity level. Messages with a severity level equal to or higher than this value are printed asynchronously. Low numbers indicate greater severity and high numbers indicate lesser severity. The all option specifies all messages are printed asynchronously, regardless of the severity level. The default value is 2.
limit number-of-buffers (Optional) Specifies the number of buffers to be queued for the terminal after which new messages are dropped. The default value is 20.

 

Command Description:

To synchronize unsolicited messages and debug output with solicited Cisco IOS software output and prompts for a specific console port line, auxiliary port line, or vty, use the logging synchronous line configuration command. To disable synchronization of unsolicited messages and debug output, use the no form of this command.

This feature is turned off by default.

If you do not specify a severity level, the default value of 2 is assumed.

If you do not specify the maximum number of buffers to be queued, the default value of 20 is assumed.

When synchronous logging of unsolicited messages and debug output is turned on, unsolicited Cisco IOS software output is displayed on the console or printed after solicited Cisco IOS software output is displayed or printed. Unsolicited messages and debug output is displayed on the console after the prompt for user input is returned. To keep unsolicited messages and debug output from being interspersed with solicited software output and prompts. After the unsolicited messages are displayed, the console displays the user prompt again.

When specifying a severity level number, consider that for the logging system, low numbers indicate greater severity and high numbers indicate lesser severity.

When a message queue limit of a terminal line is reached, new messages are dropped from the line, although these messages might be displayed on other lines. If messages are dropped, the notice "%SYS-3-MSGLOST number-of-messages due to overflow" follows any messages that are displayed. This notice is displayed only on the terminal that lost the messages. It is not sent to any other lines, any logging servers, or the logging buffer.


Example:

In the following example, line 4 is identified and synchronous logging for line 4 is enabled with a severity level of 6. Then another line, line 2, is identified and the synchronous logging for line 2 is enabled with a severity level of 7 and is specified with a maximum number of buffers to be 70,000.

Router(config)# line 4
Router(config-line)# logging synchronous level 6
Router(config-line)# line 2
Router(config-line)# logging synchronous level 7 limit 70000


Misconceptions:

Caution By configuring abnormally large message queue limits and setting the terminal to "terminal monitor" on a terminal that is accessible to intruders, you expose yourself to "denial of service" attacks. An intruder could carry out the attack by putting the terminal in synchronous output mode, making a Telnet connection to a remote host, and leaving the connection idle. This could cause large numbers of messages to be generated and queued, and these messages would unlikely consume all available RAM. You should guard against this type of attack through proper configuration.


Related Commands:

line
logging on

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