H3C Technologies H3C S7500E Series Switches - Manual (Page 354)

13-3
Every IPv6 PIM enabled interface on a router sends hello messages periodically, and thus
learns the IPv6 PIM neighboring information pertinent to the interface.
SPT establishment
The process of constructing an SPT is the “flood and prune” process.
1) In an IPv6 PIM-DM domain, an IPv6 multicast source first floods IPv6 multicast packets
when it sends IPv6 multicast data to IPv6 multicast group G: The packet is subject to an
RPF check. If the packet passes the RPF check, the router creates an (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.
z An (S, G) entry contains the multicast source address S, IPv6 multicast group address G,
outgoing interface list, and incoming interface.
z 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 13-1 , 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.