Which multicast mode sends a prune to the source when there are no local members or downstream neighbors?

PIM dense mode (PIM-DM) operates in a "flood and prune" manner. It initially floods multicast traffic to all routers in the domain. If a router has no local group members or downstream neighbors requesting the traffic, it sends a prune message back towards the source to stop receiving that specific stream. This makes PIM-DM suitable for environments where multicast receivers are densely distributed. PIM sparse mode (PIM-SM) uses a "rendezvous point" (RP) and explicitly joins multicast groups, only forwarding traffic to branches that have explicitly requested it, thus it doesn't rely on pruning from the source in the same way. IGMPv2 and IGMPv3 are Internet Group Management Protocol versions used by hosts to signal group membership to their local router, not for router-to-router pruning towards the source.