Allied Telesis AT-SBx81CFC960 - Manual (Page 275)

SwitchBlade x8106 Chassis Switch and AT-SBx81CFC960 Card Installation Guide
Section III: Building a Stack with VCStack Plus 275
transceiver in SFP+ slot 2). However, the bandwidth of the trunk is
reduced.
Note
Do not install the stacking transceivers in the SFP+ slots on the AT-
SBx81CFC960 Controller Fabric Cards until after you have activated
the stacking feature on the two chassis.