Source distribution trees
The simplest multicast distribution tree is a source tree. When a router creates a source distribution tree, it puts the multicast source as the root of the tree and creates a spanning tree through the network to each device in the group of receivers.
The source distribution tree represents the shortest path from the source to each multicast group member. For this reason, it is also a shortest path tree (SPT). Because it represents an optimal path, a source distribution tree minimizes the latency in the network. At the same time, the multicast router must track all sources and maintain state information for each source. As a result, source trees can become a burden to the multicast router, especially as the number of sources grows.