Core Based Trees (CBT)
CBT constructs a single, shared delivery tree (not source based) for mutlicast group members. This is centered around a core router (much like PIM's RP). Unlike PIM, the Shared tree is bi-directional ie traffic is sent and received over the same tree, regardless of source(s).
If the first hop router for a source is already on the tree, it forwards the multicast packets out all branches of the tree. If the first hop router for a source is not on the shared tree, a single copy of the multicast data is sent through the core router to receivers.
Regardless of the location and number of sources, group members always receive multicast data through the shared tree. Routers send join message to core an dofrm branch of the tree, suppressing downstream join messages. Downstream routers are connected to the Shared Tree through on-tree routers.
The key benefits are that a reduced amount of multicast state information is stored in routers (always send and receive over same distribution tree). Also, the traffic is aggregated onto smaller subset of links. As such, it is suitable for inter and intra domain multicast routing as there is no necessity to flood the network.
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