3Com Switch 4800G 24-Port Configuration Manual page 633

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How PIM-SM Works
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When a receiver is interested in the multicast data addressed to a specific
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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
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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.
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
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DR election
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RP discovery
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RPT building
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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 exactly the same neighbor discovery mechanism as PIM-DM does.
Refer to "Neighbor discovery" on page 630.
DR election
PIM-SM also uses hello messages to elect a designated router (DR) for a
multi-access 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
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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 DR before receivers attached
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to this device can join multicast groups through this DR.
PIM Overview 633