BPDU Filter by its name filters out any BPDUs coming in and out of an interface. Why would you want this you may ask. This would be useful when you have a layer 2 network, which two vendors manage two sides of a layer 2 network as figure 1 shows. So you want spanning-tree to be managed separately. Another good reason would be to simply not want a port to participate in spanning tree. Of course this would have major drawback. It allows layer 2 loops to occur without detection from Spanning Tree.