H3C Technologies H3C S12500-X Series Switches - Manual (Page 158)

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IPv6 PIM-SM overview
IPv6 PIM-DM uses the flood-and-prune cycles to build SPTs for IPv6 multicast data forwarding. Although
an SPT has the shortest paths from the IPv6 multicast source to the receivers, it is built with a low efficiency
and is not suitable for large- and medium-sized networks.
IPv6 PIM-SM uses the pull mode for IPv6 multicast forwarding, and it is suitable for large-sized and
medium-sized networks with sparsely and widely distributed IPv6 multicast group members.
The basic implementation of IPv6 PIM-SM is as follows:
• IPv6 PIM-SM assumes that no hosts need IPv6 multicast data. In the IPv6 PIM-SM mode, a host must
express its interest in the IPv6 multicast data for an IPv6 multicast group before the data is
forwarded to it. IPv6 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 an IPv6 multicast group, and the IPv6 multicast data to the group is
forwarded by the RP to the receivers along the RPT.
• When a receiver expresses its interest in the IPv6 multicast data addressed to a specific IPv6
multicast group, the receiver-side designated router (DR) sends a join message to the RP for the IPv6
multicast group. The path along which the message goes hop by hop to the RP forms a branch of
the RPT.
• When an IPv6 multicast source sends IPv6 multicast data to an IPv6 multicast group, the source-side
DR must register the IPv6 multicast source with the RP by unicasting register messages to the RP. The
IPv6 multicast source stops sending register message 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
IPv6 multicast source sends subsequent IPv6 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
IPv6 multicast data reaches the receivers.
The operating mechanism of IPv6 PIM-SM is summarized as follows:
• Neighbor discovery
• DR election
• RP discovery
• Embedded RP
• RPT building
• Multicast source registration
• Switchover to SPT
• Assert
Neighbor discovery
IPv6 PIM-SM uses a similar neighbor discovery mechanism as IPv6 PIM-DM does. For more information,
see " Neighbor discovery ."
DR election
On a shared-media LAN like Ethernet, only a DR forwards IPv6 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 IPv6 multicast
source to send register messages to the RP. The receiver-side DR acts on behalf of the receiver hosts to
send join messages to the RP.