primary_db 
This parameter is only relevant for Standby Database and Split Mirror Backup.
This parameter defines the connect string to the primary database instance so that BRBACKUP can log on to the primary database.
Syntax
primary_db = <conn_name>
Default value: none
Possible value:
<conn_name>: connection name from TNSNAMES.ORA for the connect from the standby or split host to the primary (that is, production) database
Example
primary_db = C11_PRIM