In order to receive a multicast stream, routers explicitly join the stream by sending join messages to the RP. This
join message is analogous to a unicast router following a default route to a destination. Effectively, the function
of the RP is a place for multicast sources and receivers to meet.
PIM-SM is extremely memory and CPU efficient. Since the only thing that most routers need to know is how to
reach the RP, memory requirements are greatly reduced. There are several methods that can be used by routers
in a PIM-SM domain to learn where to find the RP. Probably the simplest mechanism is statically configuring
all routers to reach the RP. However, if the routers are statically configured to an RP and the RP fails, then the
multicast network is no longer functional.
Alternatively, the RP can be learned dynamically through the PIM-SM bootstrap mechanism. Since this bootstrap
mechanism is dynamic, it allows for network changes and redundancy. The PIM-SM bootstrap mechanism is
generally the recommended approach for simplicity and redundancy.
Figure A4.
Appendix D: PIM Sparse Mode
In Protocol Independent Multicast-Sparse Mode (PIM-SM), the assumption is that there are no hosts that
want the multicast traffic unless they specifically ask for it. In contrast, PIM Dense Mode (PIM-DM) assumes
downstream router membership unless it receives an explicit prune message. PIM-SM is appropriate for wide-
scale deployment for both densely and sparsely populated groups and is the optimal choice for all production
networks regardless of size and membership density.
The operation of PIM-SM centers on the use of a shared tree, with a router functioning as a rendezvous
point (RP), the root of the tree. A shared tree prevents each router from maintaining source and group state
information for every multicast source. Regardless of the number or location of multicast receivers, multicast
senders register with the RP and send a single copy of multicast data through it to the registered receivers. Also,
regardless of the location or number of sources, group members register to receive data and always receive it
through the RP.
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