This implementation of PIM sparse mode supports the following features:
PIM sparse mode resolves situations that meet one or more of the following criteria:
Sparse-mode routing protocols use shared trees. In a shared tree, sources forward multicast datagrams to a directly connected router, the designated router. The designated router encapsulates the datagram and unicasts it to an assigned RP router, which then forwards the datagram to members of multicast groups. See Figure 11.
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.
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.
When all designated routers associated with a specific RP router have switched to the SPT, the RP router sends a join/prune message toward the multicast source. When the multicast source receives this message, it stops sending multicast data through the SPT.
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