It is not necessary to back up the UNDO tablespace. Think about why you would need undo information. One reason...
is for performing consistent reads with respect to transactions. But the database is down when you perform your restore, so you will not need undo for consistent reads. Another reason you would need the UNDO tablespace is to roll back a transaction. The redo logs contain the information to roll forward and undo any uncomitted transactions. If you need to restore your database and you do not have a backup of the UNDO tablespace, at a minimum you can STARTUP MOUNT the database, create a new UNDO tablespace, change the UNDO_TABLESPACE parameter to point to the new tablespace, and then OFFLINE DROP the old tablespace.
If you are implementing a new backup strategy and your database contains an UNDO tablespace, then you must be running either Oracle 9i or 10g. In either case, I strongly recommend that you use RMAN for your backup and recovery needs. RMAN has many more features than User-Managed Backups, which is what you are implementing. I would prefer RMAN to a User-Managed Backup solution almost every single time.
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