3Com Switch 8800 Advanced Software V5 Configuration Guide

PIM Overview 567
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.
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Multicast traffic is duplicated only where the distribution tree branches. This
process automatically repeats until the multicast traffic reaches the receivers.
How PIM-SM Works The working mechanism of PIM-SM is summarized as follows:
■ Neighbor discovery
■ DR election
■ RP discovery
■ RPT formation
■ Multicast source registration
■ Switchover from RPT to SPT
■ Assert
Neighbor discovery
PIM-SM uses exactly the same neighbor discovery mechanism as PIM-DM does.
Refer to
“Neighbor discovery” on page 564 described above.
DR election
PIM-SM also uses hello messages to elect a designated router (DR) for a
multi-access network (such as an LAN). 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.
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■ 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 any IGMPv1 router exists in a PIM-DM domain, a DR must be
elected to act as the IGMPv1 querier.