I do not understand why PIM DM router creates (*,G) when it receives multicats traffic.
can someone explained shortly for me please ?
This is how Cisco decided to impement it, that is the parent entry. Read the "PIM-DM State Rules" from "Developing IP Multicast Networks" , page 218 for more details. It seems that with new 15.x codes, the (*,G) will no longer be required.
can PIM DM work as shared tree instead of source tree !?
or it has to be always source tree !?
It cannot, because the shared tree is rooted at the RP as soon as there is
a client in the network. DM does not use RP.
As the book will state, the DM using that flood/prune mechanism so that the (*,G) can be build and the traffic can be flooded. Otherwise, the traffic would have no path or method to the hosts.