When I tried to open my secondary mounted database, ORA-01552 was thrown. Did a bit of google for this error and it was successfully resolved through the below simple steps.
*** Firstly all the rollback segements were checked.
select segment_name,tablespace_name,status from dba_rollback_segs;
*** Brought the UNDO tablespace online.
alter database datafile ‘/home/app/host/undotbs01.dbf’ online;
*** Set the undo management to AUTO. You will need to restart the DB.
alter system set undo_management=AUTO scope=spfile;
*** Brough the rollback segments to online.
ALTER ROLLBACK SEGMENT “_SYSSMU1_99961370$” online;