One embodiment of the present invention provides a switch. The switch includes a port that couples to a server hosting a number of virtual machines. The switch also includes a set of virtual port grouping information and a virtual port grouping mechanism. During operation, the virtual port grouping mechanism determines whether a frame is traveling from a virtual port group to the same virtual port group. If the frame is not traveling to the same virtual port group, the virtual port grouping mechanism prevents the frame from being forwarded.