FAS2750 models have internal storage so in order for them to share disks this loop is necessary. They use storage cables which are slightly different from mini SAS.
Cluster Interconnect cables
Part number X6566B-05-R6 (112-00297) or X6566B-2-R6 (112-00299). You can reference the install manual for this specs. The backplane connection is solely for HA connection, not a cluster connection.