Slots 1 to 4 are for the ethernet line cards. the cards may be installed in any order or variety in the slots. slots 5 and 6 are for the at-sbx81cfc400 controller fabric card. the chassis must be at least one controller card. you may add a second controller to add redundancy or to increase the available backplane bandwidth of the chassis. the chassis has a backplane bandwidth of 40 gbps per slot with one controller card and 80 gbps per slot with two cards.