MBONE: Multicasting Tomorrow's Internet
The intent is that when a new regional network wants to join in, they will make a request on the appropriate MBONE list, then the participants at "close" nodes will answer and cooperate in setting up the ends of the appropriate tunnels. To keep fanout down, sometimes this will mean breaking an existing tunnel to inserting a new node, so three sites will have to work together to set up the tunnels.
To know which nodes are "close" will require knowledge of both the MBONE logical map and the underlying physical network topology, for example, the physical T3 NSFnet backbone topology map combined with the network providers' own knowledge of their local topology.
Within a regional network, the network's own staff can independently manage the tunnel fanout hierarchy in conjunction with end-user participants. New end-user networks should contact the network provider directly, rather than the MBONE list, to get connected.