The Cisco IOS configuration archive is intended to provide a mechanism to store, organize, and manage an archive of Cisco IOS configuration files to enhance the configuration rollback capability provided by the configurereplace command. Before this feature was introduced, you could save copies of the running configuration using the copyrunning-configdestination-url command, storing the replacement file either locally or remotely. However, this method lacked any automated file management. On the other hand, the Configuration Replace and Configuration Rollback feature provides the capability to automatically save copies of the running configuration to the Cisco IOS configuration archive. These archived files serve as checkpoint configuration references and can be used by the configurereplace command to revert to previous configuration states.

The archiveconfig command allows you to save Cisco IOS configurations in the configuration archive using a standard location and filename prefix that is automatically appended with an incremental version number (and optional timestamp) as each consecutive file is saved. This functionality provides a means for consistent identification of saved Cisco IOS configuration files. You can specify how many versions of the running configuration are kept in the archive. After the maximum number of files are saved in the archive, the oldest file is automatically deleted when the next, most recent file is saved. The showarchive command displays information for all configuration files saved in the Cisco IOS configuration archive.

The Cisco IOS configuration archive, in which the configuration files are stored and available for use with the configurereplace command, can be located on the following file systems:

• If your platform has disk0--disk0:, disk1:, ftp:, pram:, rcp:, slavedisk0:, slavedisk1:, or tftp:

• If your platform does not have disk0--ftp:, http:, pram:, rcp:, or tftp:

The configurereplace 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 configurereplace 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 copyrunning-configdestination-url command), or, if generated externally, the replacement file must comply with the format of files generated by Cisco IOS devices. When the configurereplace 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 showarchiveconfigdifferences 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 copysource-urlrunning-config command is often used to copy a stored Cisco IOS configuration file to the running configuration. When using the copysource-urlrunning-configcommand as an alternative to the configurereplacetarget-urlcommand, the following major differences should be noted:

• The copysource-urlrunning-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 configurereplacetarget-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 copysource-urlrunning-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 configurereplacetarget-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 copysource-urlrunning-config command, whereas a complete Cisco IOS configuration file must be used as the replacement file for the configurereplacetarget-url command.

In Cisco IOS Release 12.2(25)S and 12.3(14)T, a locking feature for the configuration replace operation was introduced. When the configurereplace command is used, the running configuration file is locked by default for the duration of the configuration replace operation. This locking mechanism prevents other users from changing the running configuration while the replacement operation is taking place, which might otherwise cause the replacement operation to terminate unsuccessfully. You can disable the locking of the running configuration by using the nolock keyword when issuing the configurereplacecommand.

The running configuration lock is automatically cleared at the end of the configuration replace operation. You can display any locks that may be currently applied to the running configuration using the showconfigurationlock command.

Note

In a scenario when you are performing a configuration replace using a configuration that is not sourced from IOS(such as a custom written configuration) if the login banner has a delimiter that is not the EXT character (ASCII code 003), the banner configuration is rejected and not included in the replaced configuration. Non-working delimiters include ^C, %, #, CC etc.

The concept of rollback comes from the transactional processing model common to database operations. In a database transaction, you might make a set of changes to a given database table. You then must choose whether to commit the changes (apply the changes permanently) or to roll back the changes (discard the changes and revert to the previous state of the table). In this context, rollback means that a journal file containing a log of the changes is discarded, and no changes are applied. The result of the rollback operation is to revert to the previous state, before any changes were applied.

The configurereplace command allows you to revert to a previous configuration state, effectively rolling back changes that were made since the previous configuration state was saved. Instead of basing the rollback operation on a specific set of changes that were applied, the Cisco IOS configuration rollback capability uses the concept of reverting to a specific configuration state based on a saved Cisco IOS configuration file. This concept is similar to the database idea of saving a checkpoint (a saved version of the database) to preserve a specific state.

If the configuration rollback capability is desired, you must save the Cisco IOS running configuration before making any configuration changes. Then, after entering configuration changes, you can use that saved configuration file to roll back the changes (using the configurereplace target-url command). Furthermore, since you can specify any saved Cisco IOS configuration file as the replacement configuration, you are not limited to a fixed number of rollbacks, as is the case in some rollback models based on a journal file.

• Allows you to revert to a previous configuration state, effectively rolling back configuration changes.

• Allows you to replace the current running configuration file with the startup configuration file without having to reload the router or manually undo CLI changes to the running configuration file, therefore reducing system downtime.

• Allows you to revert to any saved Cisco IOS configuration state.

• Simplifies configuration changes by allowing you to apply a complete configuration file to the router, where only the commands that need to be added or removed are affected.

• When using the configure replace command as an alternative to the copy source-url running-config command, increases efficiency and prevents risk of service outages by not reapplying existing commands in the current running configuration.