Every router has two different configurations to consider:
The router's active configuration, which is stored in the router's RAM. Every configuration command you give is stored in the running configuration . If you reboot your router, this configuration is lost. If you make changes that you want to save, you must copy the running configuration to a safe location, such as a network server, or save it as the router's startup configuration .
The configuration that is loaded when the router boots. This configuration is stored in the router's nonvolatile memory (NVRAM) . You cannot edit a startup configuration directly. All commands you enter are stored in the running configuration, which can be copied into the startup configuration.
In other words, when you boot a router, the startup configuration becomes the initial running configuration. As you modify the configuration, the two diverge: the startup configuration remains the same, while the running configuration reflects the changes you have made. If you want to make your changes permanent, you must copy the running configuration to the startup configuration.
The following command copies the router's current running configuration into the startup configuration:
Router#copy running-config startup-config Building configuration...
Similarly, to save the running configuration on a network server using TFTP, you would give the command:
Router#copy running-config tftp
You'll be prompted for additional information, such as the remote host and the name for the saved file.
The terms "running configuration" and "startup configuration" were added in recent versions of IOS. In earlier versions, you used the command write terminal to display the current router configuration and write memory to store the current configuration. This terminology is outdated; use the copy command.