3Com Switch 4800G 24-Port Configuration Manual page 630

630 C 47: PIM C
HAPTER ONFIGURATION
How PIM-DM Works
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PIM-DM assumes that at least one multicast group member exists on each
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subnet of a network, and therefore multicast data is flooded to all nodes on
the network. Then, branches without multicast forwarding are pruned from
the forwarding tree, leaving only those branches that contain receivers. This
"flood and prune" process takes place periodically, that is, pruned branches
resume multicast forwarding when the pruned state times out and then data is
re-flooded down these branches, and then are pruned again.
When a new receiver on a previously pruned branch joins a multicast group, to
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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, namely 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 shortest path tree (SPT).
The working mechanism of PIM-DM is summarized as follows:
Neighbor discovery
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SPT building
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Graft
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Assert
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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).
Every activated interface on a router sends hello messages periodically, and thus
learns the PIM neighboring information pertinent to the interface.
SPT establishment
The process of building an SPT is the process of "flood and prune".
1 In a PIM-DM domain, when a multicast source S sends multicast data to a
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
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address G, outgoing interface list, and incoming interface.
For a given multicast stream, the interface that receives the multicast stream is
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referred to as "upstream", and the interfaces that forward the multicast stream
are referred to as "downstream".