Sub-nets with group members present can identify themselves to other routers. This report will be seen throughout the network. Multicast can be achieved by having each router forward multicast packets only on branches that lead to the destination group. If this is adopted there is a possible scaling problem. This can be prevented easily, however.
Routers can forward memberships reports in batches instead of as received.
As the internetwork grows larger the whole routing algorithm faces scaling problems. The algorithm must be able to scale itslef commensurately to the growth of the internetwork. A greater number of sub-nets and larger, more dispersed multicast groups will have to be dealt with. This can be done. A group of sub-nets can be thought of as a region.