Enable Automatic Undo Management

To enable Automatic Undo Management feature, all you have to do is set up two initialization parameters.

UNDO_TABLESPACE= undotbs1 -- Name of the undo tablespace
UNDO_MANAGEMENT= Auto -- Default is manual. Not a dynamic parameter.
UNDO_RETENTION= time_in_secs -- Default 900 secs.

From Oracle 11g,

The default value for UNDO_MANAGEMENT is AUTO (Manual in earlier releases). Also note that a null value for this parameter sets it to Automatic undo management in 11g and later, but defaults to manual in earlier releases. Hence while upgrading from previous version double check this parameter value.

Thanks