What is PIM Dense Mode?
Dense mode PIM is the older and simpler PIM mode. It works well in small networks where there are a large number of listeners, but is inefficient in larger networks. Cisco no longer recommends using dense mode. It works via a flood and prune strategy, where a multicast group is initially flooded to every router in the network, and then routers that don’t have any listeners for a group send “prune” messages back upstream to block transmission. Every three minutes the routers begin flooding again and prunes must be repeated. A later addition to dense-mode called grafting allows routers to actively request a multicast group from the upstream router.