3Com 7757 Configuration Manual page 465

Work Mechanism of
PIM-SM
In the receiving end, the router connected to the information receiver sends join
messages to the RP corresponding to the multicast group. The join message
reaches the root (namely, RP) after passing each router. The passed paths become
the branches of the rendezvous point tree (RPT).
If the sending end wants to send data to a multicast group, the first hop router
will send registration information to RP. When the registration information reaches
RP, the source tree establishing is triggered. Then the multicast source sends the
data to RP. When the data reaches RP, the multicast packets are replicated and
sent to the receiver. Replication happens only in the branch of RPT. The procedure
is repeated automatically until the multicast packets reach the receiver.
PIM-SM is independent of the special unicast routing protocol. Instead, it performs
RPF check based on the existing unicast routing table.
The working procedure of PIM-SM is:
Neighbor discovery
■
DR election
■
RP discovery
■
RPT shared tree building
■
Multicast source registration
■
Switching RPT to SPT
■
Neighbor discovery
The neighbor discovery mechanism is the same as described in PIM-DM. It is also
implemented through Hello messages sent between each router.
DR election
With the help of Hello messages, DR can be elected for the shared network, such
as Ethernet. DR will be the unique multicast information forwarder in the network.
In either the network connected to the multicast source S or the network
connected to the receiver, DR must be elected only if the network is a shared
network. The DR in the receiving end sends Join messages to RP and the DR in the
multicast source side sends Register messages to RP, as shown in Figure 114:
PIM Overview 465