Work Mechanism Of Pim-Sm - H3C S7500 Series Operation Manual

Operation Manual – Multicast
H3C S7500 Series Ethernet Switches
network bandwidth that the data packets and control messages occupy is reduced, and
the processing overhead of the router is also reduced.
In the receiving end, the router connected to the receiver sends a join message 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
a register message to the RP. When the register message 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 over the
RPT. Replication happens only in the branch of the RPT. The procedure is repeated
automatically until the multicast packets reach the receiver.
PIM-SM is independent of any specific unicast routing protocol. Instead, it performs
RPF check based on the existing unicast routing table.

7.1.4 Work Mechanism of PIM-SM

The working procedure of PIM-SM is:
Neighbor discovery
DR election
RP discovery
RPT establishing
Multicast source registration
RPT-to-SPT switchover
I. 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.
II. 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 the RP, as shown in Figure 7-3:
7-5
Chapter 7 PIM Configuration