X transmits is frame as normal and this is received by switch A. Switch A knows that X belongs to vLAN 1. If A does not know where Y is located then it will re-transmit this frame on all ports containing members of vLAN 1 and also the uplink port. However, when sending the frame over the uplink port, it will add a tag field. This tag will contain a VLAN identifier which in this case identifies the frame as belonging to vLAN1. Switch B now receives this frame, notes the tag and hence, its vLAN membership. The tag is removed. If B knows which port Y is located on then the frame will be sent to that port – providing of course, that Y is known to be a member of vLAN1. If B does not know which port Y is located on then it will send the frame to all ports that are registered to vLAN1 members.