Patent ID: 8321596

Claim:
A method for performing paired subtractive routing in a Serial Attached Small Computer System Interface (SAS) cascaded topology, comprising: assigning an input port to a SAS expander device; assigning an output port to the SAS expander device; defining the output port and the input port to be paired with each other as a primary subtractive port; programming only a SAS initiator address in the route table of the SAS expander device; adding a second input port and a second output port to the SAS expander device; defining the second input port and the second output port to be paired with each other as a secondary subtractive port; sending an OPEN command out the output port upon receiving the OPEN command into the input port if the DEST of the OPEN command is not a direct attached device of the SAS expander device and the DEST is not in the route table of the SAS expander device; sending the OPEN command out the input port upon receiving the OPEN command into the output port if the DEST of the OPEN command is not a direct attached device of the SAS expander device and the DEST is not in the route table of the SAS expander device; sending the OPEN command out the second output port upon receiving the OPEN command into the second input port if the DEST of the OPEN command is not a direct attached device of the SAS expander device and the DEST is not in the route table of the SAS expander device; sending the OPEN command out the second input port upon receiving the OPEN command into the second output port if the DEST of the OPEN command is not a direct attached device of the SAS expander device and the DEST is not in the route table of the SAS expander device; and specifying the subtractive ports of the SAS expander device via a modified routing attribute of a vendor unique SAS Management Protocol (SMP) function, wherein when the modified routing attribute exists for a port, a SAS initiator ignoring a SMP DISCOVER response attribute for the port and instead using the modified routing attribute for the port, the secondary subtractive port operates for communication with the direct attached device of the SAS expander device.