Hello Terry
Yes, you have understood it correctly. Here’s a brief explanation for more clarity:
PIM-Dense Mode: In PIM-DM, the multicast traffic is initially flooded to all parts of the network (Source Tree topology). Non-wanting parts of the network then prune back the traffic. This is called a “push” model. PIM-DM primarily uses the source tree topology, more commonly called the Shortest Path Tree (SPT), with each source having a separate distribution tree.
PIM-Sparse Mode: PIM-SM initially sends no traffic until a receiver on a network segment indicates it is interested in receiving the traffic. The traffic is then sent to that network segment by the RP (Rendezvous Point) in a Shared Tree topology. This is called a “pull” model. PIM-SM typically uses the shared tree topology, most often called a Root Point Tree (RPT) initially. This delivers greater simplicity of implementation and lower overhead. When a host wants to receive the multicast traffic, it sends a join message upstream toward the RP. Once traffic for a group is flowing, if the volume of traffic and the number of receivers warrant it, PIM-SM can switch over to the source tree topology for greater efficiency.
I hope this has been helpful!
Laz