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).