9.3 Routing Multicast Traffic
9.3.4
Shared distribution tree
Unlike source trees that have their roots at the source, share trees use a single common root placed at some chosen point in the network. Depending on the multicast routing protocol, this root is often called a rendezvous point (RP) or core, which lends itself to other common names of shared trees: RP trees (RPT) or core-based trees (CBTs).

The Figure shows a shared tree for Group 224.2.2.2 with the root located at Router D. When using a shared tree, sources must send their traffic to the root for the traffic to reach all receivers.

In this example, multicast group traffic from source Hosts A and D travels to the root (Router D) and then down the shared tree to two receivers, Hosts B and C. Because all sources in the multicast group use a common shared tree, a wildcard notation written as (*,G), pronounced "star comma G," represents the tree. In this case, * means all sources, and the G represents the multicast group. Therefore, the shared tree shown in the Figure would be written (*, 224.2.2.2).