HP A8800 Configuration Manual page 386

source tree is the shortest path from the IPv6 multicast source to the receivers, it is also called shortest path
tree (SPT).
The working mechanism of IPv6 PIM-DM is summarized as follows:
Neighbor discovery
•
SPT establishment
•
Graft
•
Assert
•
Neighbor discovery
In an IPv6 PIM domain, a PIM router discovers IPv6 PIM neighbors, maintains IPv6 PIM neighboring
relationships with other routers, and builds and maintains SPTs by periodically multicasting IPv6 PIM
hello messages (hereinafter referred to as "hello messages") to all other IPv6 PIM routers on the local
subnet.
Every interface with IPv6 PIM enabled 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 an 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 downstream 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.
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
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
374
102, a router without any receiver