Juniper JUNOSE 11.1.X MULTICAST ROUTING, Configuration Manual

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Figure 11: PIM Sparse Mode Operation
In PIM sparse mode, an RP announces a source and establishes paths from the source
to members of a multicast group before multicasting any datagrams. RPs transmit
join messages to become part of the shared tree that enables distribution of packets
to the multicast group.
However, when a source starts multicasting datagrams, PIM sparse mode can switch
to an SRT—known in PIM sparse mode as an SPT—to improve the network's
efficiency. Although shared trees minimize the traffic in the network and the costs
associated with unnecessary transmission of data, the routes in a shared tree might
be longer than those in an SPT. See Figure 12 on page 83.
Figure 12: Shared Tree Versus SPT
The designated routers on the network determine when the source switches from a
shared tree to an SPT. A designated router switches to the SPT when it receives a
certain number of packets which you can configure.
Overview
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Chapter 3: Configuring PIM for IPv4 Multicast