Network devices depend on two types of software for their operation: operating system and configuration. Like the operating system in any computer, the operating system facilitates the basic operation of the device's hardware components.
Configuration files contain the Cisco IOS software commands used to customize the functionality of a Cisco device. Commands are parsed (translated and executed) by the Cisco IOS software when the system is booted (from the startup-config file) or when commands are entered in the CLI while in configuration mode.
A network administrator creates a configuration that defines the desired functionality of a Cisco device. The configuration file is typically a few hundred to a few thousand bytes in size.
Types of Configuration Files
A Cisco network device contains two configuration files:
The running configuration file - used during the current operation of the device
The startup configuration file - used as the backup configuration and is loaded when the device is started
A configuration file may also be stored remotely on a server as a backup.
Startup Configuration File
The startup configuration file (startup-config) is used during system startup to configure the device. The startup configuration file or startup-config file is stored in non-volatile RAM (NVRAM). Since NVRAM is non-volatile, when the Cisco device is turned off, the file remains intact. The startup-config files are loaded into RAM each time the router is started or reloaded. Once the configuration file is loaded into RAM, it is considered the running configuration or running-config.
Once in RAM, this configuration is used to operate the network device.
The running configuration is modified when the network administrator performs device configuration. Changes to the running configuration will immediately affect the operation of the Cisco device. After making any changes, the administrator has the option of saving those changes back to the startup-config file so that they will be used the next time the device restarts.
Because the running configuration file is in RAM, it is lost if the power to the device is turned off or if the device is restarted. Changes made to the running-config file will also be lost if they are not saved to the startup-config file before the device is powered down.