How to Use Running-Config and Startup-Config in Cisco Switches and Routers for Efficient Network Management

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all about running config and startup config

In Cisco networking devices like switches and routers, two key configuration files play crucial roles in the functioning and management of the system—running-config and startup-config. Understanding these files is essential for network administrators when configuring, troubleshooting, or managing a Cisco network. Let’s explore what these files are, how they function, and their significance in Cisco device management.

What is Running-Config?

The running-config file refers to the configuration that is currently being used by the device. It resides in the RAM and is responsible for the live, active configuration of the router or switch at any given moment. Whenever you modify or make changes to the configuration of a Cisco device (whether through the command-line interface or a graphical interface), those changes are immediately reflected in the running-config.

Since the running-config is stored in volatile memory (RAM), it is lost upon reboot or power cycling of the device. This is why it is essential to save any necessary changes from the running-config to ensure they persist after a restart.

Key Features of Running-Config:

Stored in RAM: Volatile memory, so changes made are lost after reboot unless saved.
Reflects live configuration: Any real-time modifications made to the system take place in the running-config.
Easy to modify: You can easily edit or add new configurations to the running-config without rebooting the device.

What is Startup-Config?

The startup-config file, in contrast, is the configuration that is saved to NVRAM (non-volatile random-access memory). This file is loaded into the system’s memory when the device starts up or reboots. The startup-config file is essentially a backup of the configuration that you want the device to use after a reboot.

The main difference between startup-config and running-config is persistence. While running-config only lives in RAM and can be wiped upon restart, startup-config is persistent and remains intact even after the device is powered off.

Key Features of Startup-Config:

Stored in NVRAM: Non-volatile memory ensures that configurations persist after a reboot.
Used during boot: The device loads the startup-config upon startup, which defines its initial settings.
Requires manual saving: To ensure changes made to the running-config persist, you must manually copy the running-config to startup-config.

Relationship Between Running-Config and Startup-Config

Running-config and startup-config work in tandem. When a Cisco router or switch starts, it loads its startup-config file into RAM, making it the active running-config. Network administrators can then make modifications to the running-config as needed. However, these changes will only remain in effect during the current session unless saved.

To save the running-config to the startup-config, the copy command is used. This ensures that the changes are backed up and will be applied the next time the device is rebooted.

Command to copy the running-config to the startup-config is:

  copy running-config startup-config

This command writes the current active configuration to the NVRAM, ensuring that all settings persist after a reboot.

To view the contents of the startup-config file, use the following command:

  show startup-config

Similarly, to view the current running configuration, use:

  show running-config

Use Cases for Running-Config and Startup-Config

Both running-config and startup-config serve different purposes depending on the situation. Here are some typical scenarios where each of these configurations plays a vital role:

Immediate Changes: If you need to make real-time adjustments to your network without waiting for a reboot, you’ll work directly with the running-config. This is useful for making on-the-fly modifications to routing protocols, VLANs, or access lists.
Backup Configuration: The startup-config is your safety net. In cases where devices reboot unexpectedly (due to power failure or scheduled maintenance), the startup-config ensures that the system returns to a known and stable state.
Configuration Testing: Administrators often test new configurations by first making changes in the running-config. This allows them to observe the changes without committing them permanently. If the new configuration is successful, they can then save it to the startup-config. Otherwise, a simple reboot will revert the changes.
Disaster Recovery: If you accidentally make unwanted changes to the running-config, you can discard them by reloading the startup-config. This approach can restore your device to its previous state quickly without manually undoing individual configuration changes.
Saving Configurations: After completing all necessary configurations on a switch or router, it’s vital to save the running-config into the startup-config to prevent loss of configuration upon reboot.

How to Erase Startup-Config

If you need to reset the configuration and clear the startup-config, the following command can be used:

  write erase

This command clears the NVRAM, erasing the startup-config file, and can be followed by a device reboot to restore the system to its factory defaults.

Conclusion

Understanding the relationship between the running-config and startup-config in Cisco routers and switches is vital for network administrators. The running-config allows for live, real-time configuration changes, while the startup-config stores persistent settings to be applied upon reboot. Knowing how to manage these files effectively helps ensure network stability, reliable backups, and smooth disaster recovery. Always remember to save your running-config to startup-config if you want changes to persist through a reboot.