H3C Technologies H3C S12500-X Series Switches - Manual (Page 74)

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PIM-SM overview
PIM-DM uses the flood-and-prune cycles to build SPTs for multicast data forwarding. Although an SPT has
the shortest paths from the multicast source to the receivers, it is built with a low efficiency and is not
suitable for large- and medium-sized networks.
PIM-SM uses the pull mode for multicast forwarding, and it 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 multicast data. In the PIM-SM mode, a host must express its
interest in the multicast data for a multicast group before the data is forwarded to it. PIM-SM
implements multicast forwarding by building and maintaining rendezvous point trees (RPTs). An RPT
is rooted at a router that has been configured as the rendezvous point (RP) for a multicast group,
and the multicast data to the group is forwarded by the RP to the receivers along the RPT.
• When a receiver expresses its interest in the multicast data addressed to a specific multicast group,
the receiver-side designated router (DR) sends a join message to the RP for the 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 multicast data to a multicast group, the source-side DR must register
the multicast source with the RP by unicasting register messages to the RP. The multicast source stops
sending until it receives a register-stop message from the RP. When the RP receives the register
message, it 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.
Multicast data is replicated wherever the RPT branches, and this process automatically repeats until the
multicast data reaches the receivers.
The operating 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
On a shared-media LAN like Ethernet, only a DR forwards the multicast data. A DR is required in both the
source-side network and receiver-side network. A source-side DR acts on behalf of the multicast source to
send register messages to the RP, and the receiver-side DR acts on behalf of the receiver hosts to send join
messages to the RP.
PIM-DM does not require a DR. However, if IGMPv1 runs on any shared-media LAN in a PIM-DM domain,
a DR must be elected to act as the IGMPv1 querier for the LAN. For more information about IGMP, see
"
Configuring IGMP ."