The source address identifies the originator of the frame. By looking this address up within the switch’s vLAN database we can determine which vLAN it belongs to.
The destination address of the frame is also checked against the switch’s vLAN database. If the destination is located on the same vLAN as the source, then the frame will be transmitted through the port onto which the destination is attached. If the destination address is a broadcast address then the frame will be sent to all members of the vLAN to which the source belongs. If the destination address does not belong to the same vLAN as the source then the frame cannot be re-transmitted. The switch cannot pass traffic between vLANs – they are logically separate.