H3C Technologies H3C S10500 Series Switches - Manual (Page 145)

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unicast until it receives a register-stop message from the RP. The arrival of a register message at the
RP triggers the establishment of an SPT. Then, the multicast source sends subsequent multicast
packets along the SPT to the RP. After 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.
The working mechanism of PIM-SM is summarized as follows:
• Neighbor discovery
• DR election
• RP discovery
• RPT building
• Multicast source registration
• Switchover to SPT
• Assert
Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. For more information, see
“ Neighbor discovery .”
DR election
PIM-SM also uses hello messages to elect a DR for a multi-access network (such as Ethernet). The elected
DR will be the only multicast forwarder on this multi-access network.
A DR must be elected in a multi-access network, no matter this network connects to multicast sources or
to receivers. The receiver-side DR sends join messages to the RP. The source-side DR sends register
messages to the RP.
NOTE:
•
A DR is elected on a multi-access subnet by means of comparison of the priorities and IP addresses
carried in hello messages. An elected DR is substantially meaningful to PIM-SM. PIM-DM itself does not
require a DR. However, if IGMPv1 runs on any multi-access network in a PIM-DM domain, a DR must be
elected to act as the IGMPv1 querier on that multi-access network.
• IGMP must be enabled on a device that acts as a receiver-side DR before receivers attached to this
device can join multicast groups through this DR.
• For more information about IGMP, see the chapter “IGMP configuration.”