From official System Configuration Guide: “All NetApp single-controller systems are required to use dual-path connectivity with the exception of the FAS2040 using SAS storage shelves (due to the single SAS data port on the system).”
ACP (yellow)
SAS (red)
Connectivity for FAS2040 Single controller with Single SAS port. If there is only one Disk shelf, bottom IOM doesn't need to be connected.
ACP cabling is optional but not having or failure of ACP cabling doesn't interrupt the filer from serving Data. Its always recommended to have the ACP cabling as it helps to recover from Shelf failures, debug and identify the shelf module issues.
Refer Data ONTAP® 8.1SAN Administration Guide For 7-Mode, Storage Provisioning for SAN pg......... 8