3Com S7906E - Switch, Configuration Manual

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To facilitate description, a network comprising PIM-capable routers is referred to as a “PIM domain” in
this document.
Introduction to PIM-DM
PIM-DM is a type of dense mode multicast protocol. It uses the “push mode” for multicast forwarding,
and is suitable for small-sized networks with densely distributed multicast members.
The basic implementation of PIM-DM is as follows:
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PIM-DM assumes that at least one multicast group member exists on each 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.
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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, 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).
How PIM-DM Works
The working mechanism of PIM-DM is summarized as follows:
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Neighbor discovery
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SPT building
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Graft
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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).
Every PIM-enabled 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”.