logging synchronous
Prevents console and VTY line output from being interrupted by unsolicited system messages, ensuring that command output remains readable.
logging synchronousWhen to Use This Command
- When configuring a router via console and frequent syslog messages disrupt your typing
- During troubleshooting where you need to see clean command output without interleaved log messages
- In production environments where multiple administrators are logged in and generating syslog traffic
- When using Telnet/SSH and want to avoid message corruption of command output
Command Examples
Enable logging synchronous on console line
Router(config)# line console 0
Router(config-line)# logging synchronousRouter(config-line)# *Mar 1 00:05:23.123: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up Router(config-line)#
The command is entered without any immediate output. The next line shows a syslog message that would have interrupted typing, but with logging synchronous enabled, the prompt is redisplayed after the message, keeping your command entry intact.
Verify logging synchronous configuration
Router# show running-config | include line con 0|logging synchronousline con 0 logging synchronous
The output confirms that logging synchronous is configured under line con 0. The 'show running-config' command filters to show only the relevant lines.
Understanding the Output
The 'logging synchronous' command itself produces no direct output when entered. Its effect is observed during operation: when a syslog message is generated while you are typing a command, the message is displayed, and then the command line you were typing is automatically re-displayed. This prevents the message from corrupting your input. To verify the configuration, use 'show running-config' and look for 'logging synchronous' under the line configuration (e.g., line con 0, line vty 0 4). If present, the feature is active. There are no 'good' or 'bad' values; it is either enabled or disabled. Watch for missing 'logging synchronous' on console or VTY lines if users report interrupted command entry.
CCNA Exam Tips
CCNA exam tip: Remember that 'logging synchronous' is configured under line configuration mode, not global config.
CCNA exam tip: It only affects the line it is configured on (console, aux, or VTY).
CCNA exam tip: This command does not stop syslog messages; it just re-displays your input after the message.
CCNA exam tip: Often tested alongside 'exec-timeout' and 'logging console' for line configuration scenarios.
Common Mistakes
Mistake 1: Configuring 'logging synchronous' under global configuration mode instead of line configuration mode — the command will be rejected.
Mistake 2: Forgetting to apply it to VTY lines when only console is configured — remote users still experience interruptions.
Mistake 3: Assuming it suppresses syslog messages — it only re-displays your input, messages still appear.
Related Commands
exec-timeout [min] [sec]
Sets the inactivity timeout for an EXEC session on a line, automatically logging out idle users to free up resources and enhance security.
line vty 0 4
Enters line configuration mode for virtual terminal (VTY) lines 0 through 4 to configure remote access settings like Telnet/SSH, ACLs, and timeout parameters.
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