Just highlighting some myths we presume exist, but it is not
NBU does supports Bi-Directional Auto Image Replication
=> One of fantastic feature from Sym Team (i'm loving it), Yes with NBU it is possible to achieve it by means of configuring SLP and adding Both Storage server credentials amongst each other properties (Replication).
You can configure SLP to replicate newer completed backup images from Domain A to Domain B after certain time threshold (Default is 30 Mins, For more tuning parameters info Cheatsheet shuld be helpful enough https://www-secure.symantec.com/connect/articles/storage-life-cycle-cheat-sheet)
You can have similar SLP configured at Domain B to replicate newer succcessful images to Domain A
But keep in mind, SLP would enforce replication only with new scheduled backups , it would not replicate existing backup images lying under MSDP at both Domain. If in case you need them too replicated, Go for Rehydration method (Duplicate to Tape, Restore them back and create new backup policy with SLP as STU... looks bit lengthier process ;-)
AIR benefits when Domain A goes down by any chance, you have Catalog and Backup image ready at Domain B to restore (During Replication process backup images are automatically cataloged at target Domain, We can further have copies duplicated to Tape drive , but this has to be specified in SLP operation. Read DeDupe Admin Guide and How To for more Info)
Most important thing to highlight. If Domain A or Domain B had disaster you would be lucky enough to have another copy at Target Domain (A or B), but once Source Domain is Up and running, You would need to go through Rehydretion process mentioned above to get Target Domain backup images at Source Domain As Presently NBU does not offers Reverse Direction AIR (As stated in beginning AIR SLP implies only to newer schedules backups , and does not to existing lying backup images so) Design your strategy wisely. For more information You can also look at Performance and Tuning Guide and DeDupe Guide.
Hope this helps :-)
