---
Introduction to Cisco Console Logging
Console logging in Cisco devices involves the transmission of messages, alerts, and logs to the device's console port, typically accessed via a terminal or terminal emulator software. These logs provide real-time information about system operations, errors, configuration changes, and other critical events.
By default, Cisco IOS devices generate a variety of log messages, which can be invaluable for diagnosing issues or monitoring network health. However, the manner in which these messages are displayed on the console can impact usability, especially when administrators are typing commands or configuring devices.
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Understanding Logging Modes in Cisco Devices
Cisco devices offer different logging modes to control how messages are displayed on the console:
Synchronous Logging
- Ensures that log messages do not interrupt or interfere with the command input process.
- Displays messages in a manner that is synchronized with the command prompt, maintaining command line integrity.
- Ideal for active configuration sessions where real-time messages are necessary but should not disrupt command input.
Asynchronous Logging
- Log messages are sent independently of the command prompt.
- Messages may appear while the administrator is typing, potentially disrupting command input.
- Useful for high-priority alerts that require immediate attention, regardless of ongoing input.
Summary of Logging Modes
| Mode | Behavior | Use Case |
|------------------|--------------------------------------------------------|------------------------------------------------|
| Synchronous | Messages appear without interrupting command input | Routine monitoring during configuration |
| Asynchronous | Messages can interrupt ongoing input | Critical alerts or high-priority notifications |
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What is Synchronous Logging?
Definition and Purpose
Synchronous logging in Cisco refers to a mode where system messages are displayed in a way that aligns with the command prompt, preventing interruptions during configuration or command input. This mode ensures that log messages are printed in a manner that does not interfere with the administrator's ongoing commands.
The primary goal of synchronous logging is to improve usability during active sessions by displaying logs in a non-intrusive manner. It allows for real-time monitoring while maintaining a smooth command-line experience.
Why Use Synchronous Logging?
- To prevent log messages from interrupting commands being typed.
- To maintain clarity and focus during configuration tasks.
- To improve the overall user experience, especially in environments with frequent log messages.
- To ensure important messages are still visible without disrupting workflow.
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Configuring Synchronous Logging on Cisco Devices
Configuration of console logging modes involves specific commands entered in privileged EXEC or global configuration modes.
Enabling Synchronous Logging on Console
The common method to enable synchronous logging is via the `logging synchronous` command applied to the console line.
Step-by-step configuration:
1. Access privileged EXEC mode:
```bash
enable
```
2. Enter global configuration mode:
```bash
configure terminal
```
3. Enter the console line configuration:
```bash
line console 0
```
4. Enable synchronous logging:
```bash
logging synchronous
```
5. Exit configuration mode:
```bash
end
```
Note: The `logging synchronous` command is typically set per line, which allows administrators to tailor the behavior per terminal connection.
Verifying the Configuration
To confirm whether the `logging synchronous` command is active:
```bash
show running-config | include logging synchronous
```
Or specifically check the console line:
```bash
show running-config | section line
```
If configured, you should see a line similar to:
```plaintext
line console 0
logging synchronous
```
---
Additional Considerations in Console Logging
Logging Levels
Cisco devices categorize log messages into different severity levels, from 0 (emergencies) to 7 (debugging). Administrators can filter the logs to display only messages of certain severity levels, reducing clutter and focusing on critical issues.
Common levels include:
- Level 0: Emergency – System is unusable.
- Level 1: Alert – Immediate action required.
- Level 2: Critical – Critical conditions.
- Level 3: Error – Error conditions.
- Level 4: Warning – Warning conditions.
- Level 5: Notification – Normal but significant conditions.
- Level 6: Informational – Informational messages.
- Level 7: Debugging – Debugging messages.
Configuring the logging level can be done with the command:
```bash
logging trap
```
For example, to include informational messages:
```bash
logging trap informational
```
Balancing Visibility and Distraction
While logging is essential, excessive messages can overwhelm administrators, especially during active configurations. Using synchronous logging helps, but further filtering by severity levels ensures only necessary messages are displayed.
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Best Practices for Using Synchronous Logging
1. Always Configure Synchronous Logging on Console Lines
Enabling `logging synchronous` on console lines ensures that log messages do not interfere with command inputs, which is particularly useful during troubleshooting and configuration.
2. Combine with Appropriate Logging Levels
Adjust logging levels to prevent unnecessary messages from cluttering the console, especially in production environments.
3. Use Logging Buffers and Logs for Historical Data
While console logging provides real-time information, it is advisable to also configure buffered logging:
```bash
logging buffered 4096
```
This command stores logs in memory for later review.
4. Monitor Critical Events Separately
Set up alerts or SNMP traps for critical events to ensure they are addressed promptly without overwhelming the console.
5. Regularly Review Log Configuration
Network environments evolve, and so should logging strategies. Regularly review and adjust logging configurations to match operational needs.
---
Common Troubleshooting Scenarios
Scenario 1: Log Messages Disrupt Command Input
- Cause: The device is configured with asynchronous logging, causing messages to appear mid-command.
- Solution: Enable synchronous logging with `logging synchronous` on the console line.
Scenario 2: Critical Logs Are Not Visible in Console
- Cause: Logging level is set too high, filtering out critical messages.
- Solution: Adjust logging levels using `logging trap` or `logging monitor` commands and verify with `show logging`.
Scenario 3: Excessive Log Messages on Console
- Cause: High volume of logs due to debug or verbose levels.
- Solution: Reduce logging verbosity and use buffered logs for detailed analysis.
---
Advanced Topics and Additional Features
Using Logging to Different Destinations
Cisco devices support multiple logging destinations beyond the console:
- Logging to buffer: Stores logs in memory.
- Logging to syslog servers: Sends logs to remote servers for centralized management.
- Logging to monitor sessions: Displays logs during active sessions.
Configuring Remote Logging
To send logs to a syslog server:
```bash
logging host
logging trap
```
Ensure network connectivity and proper filtering are configured for security.
Enhancing Log Management with SNMP
Simple Network Management Protocol (SNMP) traps can be used to alert administrators about critical events without relying solely on console logs.
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Conclusion
Effective management of console logging on Cisco devices, especially through the use of synchronous logging, is vital for maintaining a smooth and efficient network environment. By understanding how to configure and optimize logging modes, administrators can ensure that important system messages are visible without disrupting ongoing tasks. Proper use of logging levels, destinations, and filters further enhances visibility and responsiveness.
In summary:
- Synchronous logging prevents message interruptions during configuration.
- It can be configured easily via the `logging synchronous` command.
- Combining synchronous logging with appropriate severity levels and remote logging ensures comprehensive and manageable log monitoring.
- Regular review and adjustment of logging configurations are essential as network environments change.
Mastering console logging strategies empowers network administrators to troubleshoot effectively, maintain operational awareness, and ensure network stability.
---
Frequently Asked Questions
What does the 'logging synchronous' command do on Cisco consoles?
The 'logging synchronous' command prevents log messages from interrupting your command input by temporarily hiding them, making command line editing smoother on Cisco devices.
How do I enable 'logging synchronous' on a Cisco terminal session?
You can enable it by entering global configuration mode with 'line vty 0 4' or 'line console 0', then typing 'logging synchronous'.
Why is 'logging synchronous' recommended for Cisco console access?
It improves user experience by preventing system messages from overlapping or interrupting command input, ensuring cleaner and more manageable sessions.
Can I apply 'logging synchronous' to all lines simultaneously?
Yes, by configuring each line (console, vty, auxiliary) individually with the 'logging synchronous' command, or by scripting configuration commands across multiple lines.
Is 'logging synchronous' only useful for console sessions or also for SSH sessions?
It is useful for both console and remote sessions (SSH, Telnet), as it helps manage log messages during command input regardless of the connection type.
How do I verify if 'logging synchronous' is enabled on a Cisco device?
You can check the configuration with 'show run | include logging synchronous' or inspect individual line configurations with 'show run | section line'.
What issues can occur if 'logging synchronous' is not enabled?
Without it, log messages may appear in the middle of command input, causing confusion, command misinterpretation, or difficulty in entering commands accurately.
Can I disable 'logging synchronous' after enabling it on a line?
Yes, you can disable it by entering line configuration mode and removing the command with 'no logging synchronous'.
Are there any performance considerations when using 'logging synchronous'?
No significant performance impact; it is primarily a user interface feature to improve readability and command input experience on Cisco devices.
