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forwarding when the pruned state times out. Data is then flooded again down these branches, and
then the branches are pruned again.
• When a new receiver on a previously pruned branch joins a multicast group, to reduce the join
latency, PIM-DM uses a graft mechanism to resume data forwarding to that branch.
Generally speaking, the multicast forwarding path is a source tree. That is, it is a forwarding tree with the
multicast source as its "root" and multicast group members as its "leaves." Because the source tree is the
shortest path from the multicast source to the receivers, it is also called a shortest path tree (SPT).
The working mechanism of PIM-DM is summarized as follows:
• Neighbor discovery
• SPT building
• Graft
• Assert
Neighbor discovery
In a PIM domain, a PIM router discovers PIM neighbors, maintains PIM neighboring relationships with
other routers, and builds and maintains SPTs by periodically multicasting hello messages to all other PIM
routers (224.0.0.13) on the local subnet.
NOTE:
Every PIM-enabled interface on a router sends hello messages periodically, and thus learns the PIM
neighboring information pertinent to the interface.
SPT building
The process of building an SPT is the flood-and-prune process.
1. In a PIM-DM domain, when a multicast source S sends multicast data to multicast group G, the
multicast packet is first flooded throughout the domain. The router first performs RPF check on the
multicast packet. If the packet passes the RPF check, the router creates an (S, G) entry and forwards
the data to all downstream nodes in the network. In the flooding process, an (S, G) entry is created
on all the routers in the PIM-DM domain.
2. Then, nodes without receivers downstream are pruned. A router having no receivers downstream
sends a prune message to the upstream node to "tell" 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 multicast group down to this node.
An (S, G) entry contains the multicast source address S, multicast group address G, outgoing
interface list, and incoming interface.
For a given multicast stream, the interface that receives the multicast stream is referred to as
"upstream," and the interfaces that forward the multicast stream are referred to as "downstream."
A prune process is first initiated by a leaf router. As shown in Figure 40, a r
outer without any receiver
attached to it (the router connected with Host A, for example) sends a prune message. This prune process
goes on until only necessary branches are left in the PIM-DM domain. These branches constitute the SPT.
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