The router connected to the receiver sends the join message to the RP of a certain multicast
group. The path along which the join message is sent to the RP hop-by-hop forms a branch
of RPT.
When the multicast source is sending multicast data to a multicast group, the router directly
connected to the multicast source firstly registers to the RP by sending the Register
Message to the RP in unicast mode. The arrival of the register message at the RP triggers
the establishment of the SPT. Then the multicast source sends the multicast data along the
SPT to the RP. The multicast data will be duplicated and distributed to the receivers after
they arrive at the RP.
Note:
The duplicating process only takes place at the branching point of the distributing tree, and this
process automatically repeats until the packets arrives at the final receivers.
The work process of PIM SM can be generalized below:
Neighbor Discovering
DR Electing
RP Discovering
RPT Building
Multicast Source Registering
Switching from RPT to SPT
Asserting
Neighbor Discovering
The neighbor discovering mechanism of PIM SM and PIM DM is the same, for more details,
refer to
DR Electing
The DR (Designated Router) in the shared network is elected through the Hello message, and
works as the exclusive router to forward multicast data in this shared network.
Whether the network connects to the multicast source or the network connects to the
receivers, the DR must be elected if the network is a shared one. The DR is responsible for
sending join message to the RP in the receiver side and sending register message to the RP in
the multicast source side.
Note:
The DR is elected between the multiple routers of the network segment by comparing the
priorities and IP addresses carried in Hello packets. The elected DR has practical meaning
in PIM SM; with PIM DM operation, the DR has meaning only if IGMPv1 is in use, the elected
DR functions as the IGMP querier on account that IGMPv1 does not have an IGMP querier
election process.
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