3Com S7902E, Configuration Manual

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1-3
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 1-1 , 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.
Figure 1-1 SPT establishment in an IPv6 PIM-DM domain
The “flood and prune” process takes place periodically. A pruned state timeout mechanism is provided.
A pruned branch restarts multicast forwarding when the pruned state times out and then is pruned again
when it no longer has any multicast receiver.