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Center-based Trees

Rather than flooding the data everywhere, or flooding the membership information everywhere, algorithms in the center-based trees category map the multicast group address to a particular unicast address of a router, and they build explicit distribution trees centered around this particular router. There are three main problems that need to be solved to get this approach to work:

How to perform the mapping from group address to center-address?
How to choose the location of the center so that the distribution trees are efficient?
How to actually construct the tree given the center address.
Different protocols have come up with different solutions to these problems. There are three center-based tree protocols worth us exploring in a little detail because they illustrate different approaches: Core-Based Trees (CBT), Sparse-mode PIM (SM-PIM) and the Border Gateway Multicast Protocol (BGMP).