Broadcast-and-prune
Build source-based trees. Routers in the tree maintain state per (source, group) pair. Eg: DVMRP, PIM-DM
“Data-driven”: broadcast data to internetwork edges
If IGMP-derived group databases say that there are no local members, the edge routers send a “prune” message upstream. This is “soft state” stored in on-tree routers.
Off-tree routers maintain “prune” state per tree to allow “grafting” when members newly appear
OK for small intranets with densely distributed hosts,
- Likely that significant fraction of edge LANs will have group members
Disadvantages: Waste of bandwidth, State in off-tree routers, separate tree per source, even for same group => scalability