HP A8800 Configuration Manual page 126

When a multicast source sends a multicast packet to a multicast group, the source-side designated
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router (DR) 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.
The operating mechanism of PIM-SM is summarized as follows:
Neighbor discovery
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DR election
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RP discovery
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• RPT building
Multicast source registration
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Switchover from RPT to SPT
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Assert
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Neighbor discovery
PIM-SM uses a similar neighbor discovery mechanism as PIM-DM does. See
DR election
PIM-SM also uses hello messages to elect a designated router (DR) for a multi-access network (such as an
Ethernet network). 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 DR at the receiver side sends join messages to the RP; the DR at the multicast source side
sends register messages to the RP.
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 "Configuring IGMP."
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"Neighbor discovery."