Unlike sparse mode, in which data is forwarded only to routers sending an explicit request, dense mode implements a flood-and-prune mechanism, similar to DVMRP. In PIM dense mode, there is no RP. A router receives the multicast data on the interface closest to the source, then forwards the traffic to all other interfaces (see Figure 28).
Figure 28: Multicast Traffic Flooded from the Source Using PIM Dense Mode
Flooding occurs periodically. It is used to refresh state information, such as the source IP address and multicast group pair. If the router has no interested receivers for the data, and the OIL becomes empty, the router sends a prune message upstream to stop delivery of multicast traffic (see Figure 29).
Figure 29: Prune Messages Sent Back to the Source to Stop Unwanted Multicast Traffic
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