The PIM-SM will forward the multicast data packet by establishing the multicast distribution
tree. The multicast distribution tree is divided into the Shared Tree, which takes the RP of the
group G as the root, and the Shortest Path Tree, which takes the multicast source as the root.
The PIM-SM will establish and maintain the multicast distribution tree by the join/prune
mechanism explicitly. As shown in the figure above, when DR receives one Join from the
receiving terminal, it will send one (*.G) join message to the RP direction of the group G in
hop-by-hop multicast way, to join the share tree. When the source host sends the multicast
data to the group, the source data is encapsulated into the registration message, and is sent
to the RP in the unicast way by its DR, and then RP will forward the de-encapsulated data
packet of the source to various group members. RP will send (S, G) the join information to
the hop one equipment to the source direction, to add the shortest path tree of this source,
the data packet of this source will be sent to the RP along its shortest path tree without the
encapsulation. When the first multicast data reaches along this tree, RP will send the
registration stop message to the DR of the source, to make DR stop registering the
encapsulation process. Then, the multicast data of this source will not register the
encapsulation, but be sent to the RP along the shortest path tree of the source, and then
forwarded to various group members along the share tree by RP. If it doesn‘t require the
multicast data, DR will multicast prune messages to the RP of the group G hop-by-hop, so
as to prune the share tree.
The PIM-SM also concerns with the selection mechanism of the root node RP. One or more
Candidate-BSRs are configured in the PIM-SM domain. It will apply a certain rule to select
the BSR. The PIM-SM domain also configures the Candidate-RP, to send the information
packet of the address and the service multicast group in the unicast way. The BSR will
generate the BSR information which includes a system candidate RP and corresponding
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