OK so basics first
- don't use NDMP option if the Isilon contains LUNs that are presented to other systems - we cannot do any granular content of LUNs for restores - you would have to restore the whole LUN which for most customers would be a problem
- whilst the NDMP option can backup to disk/deduplication on the BE server itself, it is primarily designed for you to hang a tape library directly off the NDMP device (in your case the Isilon)
- backup over CIFS/SMB can be performed - but 1) Incremental/differential style backups are not officially supported (in some cases they might still work, in others they will always backup as a full. 2) If the 600TB you talk about is lots of smallish files you will almost certainly experience performance issues 3) The CIFS protocol is, in theory, not as efficient as the NDMP protocol for backups
You should perhaps setup a trailware against both NDMP and CIFS (not at same time as you have to disable NDMP on the Isilon to use CIFS)
BTW 600TB sounds like a huge amount of data and as such you possibly might need to look more at what NetBackup can do as it could be a more appropriate product for the quantity.