How Pim-Sm Works - H3C S7500E Series Operation Manual

Operation Manual – Multicast
H3C S7500E Series Ethernet Switches
PIM-SM is a type of sparse mode multicast protocol. It uses the "pull mode" for
multicast forwarding, and is suitable for large- and medium-sized networks with
sparsely and widely distributed multicast group members.
The basic implementation of PIM-SM is as follows:
PIM-SM assumes that no hosts need to receive multicast data. In the PIM-SM
mode, routers must specifically request a particular multicast stream before the
data is forwarded to them. The core task for PIM-SM to implement multicast
forwarding is to build and maintain rendezvous point trees (RPTs). An RPT is
rooted at a router in the PIM domain as the common node, or rendezvous point
(RP), through which the multicast data travels along the RPT and reaches the
receivers.
When a receiver is interested in the multicast data addressed to a specific
multicast group, the router connected to this receiver sends a join message to the
RP corresponding to that multicast group. The path along which the message
goes hop by hop to the RP forms a branch of the RPT.
When a multicast source sends a multicast packet to a multicast group, the router
directly connected with the multicast source first registers the multicast source
with the RP by sending a register message to the RP by unicast. The arrival of this
message at the RP triggers the establishment of an SPT. Then, the multicast
source sends subsequent multicast packets along the SPT to the RP. Upon
reaching the RP, the multicast packet is duplicated and delivered to the receivers
along the RPT.
Note:
Multicast traffic is duplicated only where the distribution tree branches, and this process
automatically repeats until the multicast traffic reaches the receivers.

5.1.4 How PIM-SM Works

The working mechanism of PIM-SM is summarized as follows:
Neighbor discovery
DR election
RP discovery
RPT building
Multicast source registration
Switchover from RPT to SPT
Assert
5-6
Chapter 5 PIM Configuration