HP 6125XLG Configuration Manual page 68

Neighbor discovery
In a PIM domain, each interface that runs PIM on a router periodically multicasts PIM hello messages to
all other PIM routers (identified by the address 224.0.0.13) on the local subnet to discover PIM neighbors,
maintain PIM neighboring relationship with other routers, and build and maintain SPTs.
SPT building
The process of building an SPT is the flood-and-prune process:
1. In a PIM-DM domain, when the multicast source S sends multicast data to the multicast group G,
the multicast data is flooded throughout the domain. A router performs an RPF check for the
multicast data. If the RPF check succeeds, the router creates an (S, G) entry and forwards the data
to all downstream nodes in the network. In the flooding process, all the routers in the PIM-DM
domain create the (S, G) entry.
2. The nodes without downstream receivers are pruned. A router that has no downstream receivers
sends a prune message to the upstream node to remove the interface that receives the prune
message from the (S, G) entry. In this way, the upstream stream node stops forwarding subsequent
packets addressed to that multicast group down to this node.
NOTE:
An (S, G) entry contains a multicast source address S, a multicast group address G, an outgoing interface
list, and an incoming interface.
A prune process is initiated by a leaf router. As shown in
have any downstream receivers initiates a prune process by sending a prune message toward the
multicast source. This prune process goes on until only necessary branches are left in the PIM-DM domain,
and these necessary branches constitute an SPT.
Figure 24 SPT building
Source
Server
The pruned state of a branch has a finite holdtime timer. When the timer expires, multicast data is again
forwarded to the pruned branch. The flood-and-prune cycle takes place periodically to maintain the
forwarding branches.
SPT
Prune message
Multicast packets
Figure 24, the router interface that does not
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Host A
Receiver
Host B
Receiver
Host C