You can switch the ICM and the SAP Web Dispatcher to maintenance mode, if you want to carry out maintenance or upgrade tasks in the SAP system or on the application server.
The maintenance mode has a different meaning for the ICM than for the Web Dispatcher.
The ICM will then log off from the message server and is no longer available for Web receiving and sending Web requests. The SAP Web dispatcher stops sending it new requests, since it gets its information about the available application servers from the message server. The ICM still processes existing requests.
If Internet users access this ICM directly from the browser, they are notified that the ICM is in maintenance mode.
You can override this page. For more information about the error code and the procedure see ICM/Web Dispatcher Error Handling
The Web Dispatcher serve as the initial point of access for users into an SAP system with several application servers. When you shut down the application server for maintenance (upgrade, backup and so on), an informative page explaining why it is not available should be displayed to the Internet user, and not simply the message, "page cannot be displayed".
For this the Web dispatcher provides the maintenance mode that displays the maintenance page (the same as with direct access to the ICM). You can set this page up to display the reason why the application server is not available and when it will be available again.
The file for the maintenance page must be called ICMERR-EMAINTENANCE.
The list of all the error codes as well as a description about working with your own error pages can be found in ICM error handling.
You can switch the ICM and the Web Dispatcher to maintenance mode using the Web Administration Interface.
With the ICM you can also use the ICM Monitor (transaction SMICM).