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IPv6 PIM-SM
IPv6 PIM-DM uses the flood-and-prune principle to build SPTs for IPv6 multicast data distribution.
Although an SPT has the shortest path, it is built with a low efficiency. Therefore, the PIM-DM mode
is not suitable for large-sized and medium-sized networks.
IPv6 PIM-SM is a type of sparse-mode IPv6 multicast protocol. It uses the pull mode for IPv6
multicast forwarding and 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:
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IPv6 PIM-SM assumes that no hosts need to receive IPv6 multicast data. In the IPv6 PIM-SM
mode, routers must specifically request a particular IPv6 multicast stream before the data is
forwarded to them. The core task for IPv6 PIM-SM to implement IPv6 multicast forwarding will
build and maintain RPTs. An RPT is rooted at a router in the IPv6 PIM domain as the common
node, or RP, through which the IPv6 multicast data travels along the RPT and reaches the
receivers.
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After a receiver joins an IPv6 multicast group, the router connected to this receiver sends a join
message to the RP corresponding to that IPv6 multicast group. The path along which the
message goes hop by hop to the RP forms a branch of the RPT.
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When an IPv6 multicast source sends IPv6 multicast streams to an IPv6 multicast group, the
source-side DR first registers the multicast source with the RP by sending register messages to
the RP by unicast until it receives a register-stop message from the RP. The arrival of a register
message at the RP triggers the establishment of an SPT. The IPv6 multicast source sends
subsequent IPv6 multicast packets along the SPT to the RP. After reaching the RP, the IPv6
multicast packet is duplicated and delivered to the receivers along the RPT.
NOTE:
IPv6 multicast traffic is duplicated only where the distribution tree branches, and this process
automatically repeats until the IPv6 multicast traffic reaches the receivers.
The operating mechanism of IPv6 PIM-SM is summarized as follows:
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Neighbor discovery
IPv6 PIM-SM uses the similar neighbor discovery mechanism as IPv6 PIM-DM does. For more
information, see "
DR election
IPv6 PIM-SM also uses hello messages to elect a DR for a multi-access network (such as a LAN).
The elected DR is the only IPv6 multicast forwarder on this multi-access network.
In the case of a multi-access network, a DR must be elected, no matter this network connects to IPv6
multicast sources or to receivers. The DR at the receiver side sends join messages to the RP. The
source-side DR sends register messages to the RP.
