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

IGMPv3 is a host-to-router signaling protocol for multicast group membership and does not directly send prune messages to the source in the same operational context as PIM modes.

PIM dense mode operates by initially flooding multicast traffic to all routers in the domain and then relies on prune messages to stop forwarding traffic to segments without active receivers. When a router in PIM dense mode has no connected group members or downstream PIM neighbors for a specific multicast group, it sends a prune message upstream towards the source, effectively removing itself from the distribution tree for that group.

PIM sparse mode explicitly builds shared trees by requiring receivers to join a Rendezvous Point (RP) and does not inherently flood and then prune from the source in the same manner as dense mode.

IGMPv2 is a host-to-router signaling protocol for multicast group membership and, like IGMPv3, does not directly send prune messages to the source in the same operational context as PIM modes.