Config replace allows you to overwrite your running config with the startup config or a config from flash, tftp, etc. WITHOUT reloading the router.

More info:
http://articles.techrepublic.com.com/5100-10878_11-6184709.html

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t7/feature/guide/gtrollbk.html

Configuration Replace

The configure replace command provides the capability to replace the current running configuration with any saved Cisco IOS configuration file. This functionality can be used to revert to a previous configuration state, effectively rolling back any configuration changes that were made since the previous configuration state was saved.

When using the configure replace command, you must specify a saved Cisco IOS configuration as the replacement configuration file for the current running configuration. The replacement file must be a complete configuration generated by a Cisco IOS device (for example, a configuration generated by the copy running-config destination-url command), or, if generated externally, the replacement file must comply with the format of files generated by Cisco IOS devices. When the configure replace command is entered, the current running configuration is compared with the specified replacement configuration and a set of diffs is generated. The algorithm used to compare the two files is the same as that employed by the show archive config differences command. The resulting diffs are then applied by the Cisco IOS parser to achieve the replacement configuration state. Only the diffs are applied, avoiding potential service disruption from reapplying configuration commands that already exist in the current running configuration. This algorithm effectively handles configuration changes to order-dependent commands (such as access lists) through a multiple pass process. Under normal circumstances, no more than three passes are needed to complete a configuration replace operation, and a limit of five passes is performed to preclude any looping behavior.

The Cisco IOS copy source-url running-config command is often used to copy a stored Cisco IOS configuration file to the running configuration. When using the copy source-url running-config command as an alternative to the configure replace target-url command, the following major differences should be noted:

•The copy source-url running-config command is a merge operation and preserves all the commands from both the source file and the current running configuration. This command does not remove commands from the current running configuration that are not present in the source file. In contrast, the configure replace target-url command removes commands from the current running configuration that are not present in the replacement file and adds commands to the current running configuration that need to be added.

•The copy source-url running-config command applies every command in the source file, whether or not the command is already present in the current running configuration. This algorithm is inefficient and, in some cases, can result in service outages. In contrast, the configure replace target-url command only applies the commands that need to be applied—no existing commands in the current running configuration are reapplied.

•A partial configuration file may be used as the source file for the copy source-url running-config command, whereas a complete Cisco IOS configuration file must be used as the replacement file for the configure replace target-url command.