Routeadmin

To replace the current running configuration with a saved Cisco IOS configuration file, use the configure replace command in privileged EXEC mode.

         Router#configure replace flash:test.cfg


This command will replace (NOTE: not merge) the running config with the config in file test.cfg. This command was introduced in 12.3(7)T


Here is an example of the effect:

         Router#sh ip int brief
          Interface               IP-Address      OK?     Method     Status     Protocol
          FastEthernet0/0    10.10.10.3      YES     manual      up           up

          FastEthernet0/1    10.1.1.1           YES    manual      up           up

          Serial0/3/0             unassigned      YES    unset        administratively down down

         Router#configure replace flash:test.cfg
          This will apply all necessary additions and deletions
          to replace the current running configuration with the
          contents of the specified configuration file, which is
          assumed to be a complete configuration, not a partial
          configuration. Enter Y if you are sure you want to proceed. ? [no]: y
          Total number of passes: 1
         Rollback Done

          r1#
          Dec 18 00:44:05.927: Rollback:Acquired Configuration lock.
          Dec 18 00:44:07.703: %PARSER-3-CONFIGNOTLOCKED: Unlock requested by process '3'.
          Configuration not locked.
          Dec 18 00:44:08.819: %LINK-5-CHANGED: Interface FastEthernet0/1, changed state to administratively down

         r1#sh ip int brief
          Interface               IP-Address      OK?     Method     Status     Protocol
          FastEthernet0/0    10.10.10.3      YES     manual      up           up

          FastEthernet0/1    10.1.1.1           YES    manual      administratively down down

          Serial0/3/0             unassigned      YES    unset        administratively down down


NOTE the following:
          1) Router name has changed
          2) FastEthernet 0/1 no longer has an IP address
          3) FastEthernet 0/1 is shutdown

This is great as rather than just MERGING a config, the command REPLACES the config. You can replace the running-config from many places:

          r1#configure replace ?
          archive: URL of config file that will replace running-config
          cns: URL of config file that will replace running-config
          flash: URL of config file that will replace running-config
          ftp: URL of config file that will replace running-config
          http: URL of config file that will replace running-config
          https: URL of config file that will replace running-config
          null: URL of config file that will replace running-config
          nvram: URL of config file that will replace running-config
          pram: URL of config file that will replace running-config
          rcp: URL of config file that will replace running-config
          scp: URL of config file that will replace running-config
          system: URL of config file that will replace running-config
          tftp: URL of config file that will replace running-config
          xmodem: URL of config file that will replace running-config
          ymodem: URL of config file that will replace running-config

         
You could thus have a backup config in flash or on a TFTP server and restore immediately to running config.