EMC, NetWorker Module for Oracle : Backup STrategy

We are in the phase of implementing EMC Networker backup for Oracle 11g realease 2. The idea is to backup the Fast Recover Area (FRA) to the tape every day.
In order to synchronize Tape catalog with the RMAN catalog; we are thinking to disable the RMAN retention policy and use the Tape Catalog. Which means; we want to use only the NetWorker server policy, and thus disable the Oracle retention policy with the following command: *configure retention policy to none*;
I believe by using this, every time we backup to tape we hav**e to crosscheck backup and delete obsolete from the tape, but, by doing this, How the Networker will delete the files and know which files are oboslete from the Oracle Catalog?
I would like to understand this phase.

Thank you,
S

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