Section 6.3.4 discusses 802.11 mobility, in which a wireless station moves from one BSS to another within

Question:

Section 6.3.4 discusses 802.11 mobility, in which a wireless station moves from one BSS to another within the same subnet. When the APs are interconnected with a switch, an AP may need to send a frame with a spoofed MAC address to get the switch to forward the frame properly. Why?