In STP 802.1D there`re some enhancements ,one of these enhancements is (Uplink fast) feature .
this feature is using (dummy multicast) frame which is working like this :-
src mac=the mac address in the CAM table
dst mac=multicast address
this is used in order to inform all the other switches that the mac addresses are inside my local CAM table could be reached via this new RP . my local switch is going to send (dummy multicast) frame upstream towards the other switches to change their CAM table information.
but if that is how the dummy multicast works, why do we need TCN messages?