UNDO_TABLESPACE should be specified for each Real Application Cluster instance

Oracle automatically manages undo segments within a specific undo tablespace that is assigned to an instance. Only the instance assigned to the undo tablespace can modify the contents of that tablespace. If the UNDO_TABLESPACE parameter is omitted, the first available undo tablespace in the database is chosen. If no undo tablespace is available, the instance will start without an undo tablespace. In such cases, user transactions will be executed using the SYSTEM rollback segment and could lead to space issues and ora-1555 errors.

The UNDO_TABLESPACE initialization parameter should be set to a non-null value. A unique value is also needed for the UNDO_TABLESPACE on each Real Application Cluster instance. Assign undo tablespaces in the Oracle RAC database by specifying a different value for the UNDO_TABLESPACE parameter for each instance in the SPFILE or individual PFILEs.

The value of the UNDO_TABLESPACE parameter can also be changed by using the alter system command (alter system set <parameter_name> = <value> scope=both).

Refer to Oracle Real Application Clusters Administration and Deployment Guide 11g Release 2 (11.2) (http://download.oracle.com/docs/cd/E11882_01/rac.112/e16795/storage.htm#i3915041).