With a non-trival number of subnets that would like to operate together as a single BACnet network, there are a variety of topologies that could be used in the relationship between BBMDs. The different configurations change the way the BBMDs communicate with each other (if at all) and the way they mantain their relationship as supervisors of the BACnet network. One basic solution is to mirror the IGMP topology and the original topology of the Internet, a star.
There are some advantages to using a star:
And there are some disadvantages: