3Com Switch 8800 Advanced Software V5 Configuration Guide

IPv6 PIM Overview 673
(S, G) entry and forwards the packet to all downstream nodes in the network. In
the flooding process, an (S, G) entry is created on all the routers in the IPv6
PIM-DM domain.
2 Then, nodes without downstream receivers are pruned: A router having no down
stream receivers sends a prune message to the upstream node to notify the
upstream node to delete the corresponding interface from the outgoing interface
list in the (S, G) entry and stop forwarding subsequent packets addressed to that
IPv6 multicast group down to this node.
n
■ An (S, G) entry contains the multicast source address S, IPv6 multicast group
address G, outgoing interface list, and incoming interface.
■ For a given IPv6 multicast stream, the interface that receives the IPv6 multicast
stream is referred to as "upstream", and the interfaces that forward the IPv6
multicast stream are referred to as "downstream".
A prune process is first initiated by a leaf router. As shown in Figure 201, a router
without any receiver attached to it (the router connected with Host A, for
example) sends a prune message, and this prune process goes on until only
necessary branches are left in the IPv6 PIM-DM domain. These branches constitute
the SPT.
Figure 201 Building an SPT in an IPv6 PIM-DM domain
The "flood and prune" process takes place periodically. The device sets a
countdown timer for each pruned interface. When the countdown timer for a
pruned interface expires, multicast traffic starts to flow to the interface again, and
then the interface is pruned again when it no longer has any multicast receiver
attached to it.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
IPv6 multicast packets
SPT
Prune message