PIM dense mode uses a reverse-path multicast, flood-and-prune mechanism. The protocol was developed for situations that meet one or more of the following criteria:
Dense-mode routing protocols use SRT algorithms. An SRT algorithm establishes a tree that connects each source in a multicast group to the members of the group. All traffic for the multicast group passes along this tree.
Figure 9 illustrates how PIM dense mode works. When a source sends a multicast packet to a first-hop router, the first-hop router multicasts that packet to its neighbors. Those neighbors in turn forward the packet to their neighbors and their hosts that belong to the multicast group. If a neighbor has no hosts that belong to the multicast group and has no other PIM neighbors, it returns a prune message to the first-hop router. The first-hop router does not multicast subsequent packets for that group to neighbors who respond with prune messages.
Figure 9: PIM Dense Mode Operation
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