Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 574)

PIM Sparse devices are organized into domains. A PIM Sparse domain is a contiguous set of devices
that all implement PIM and are configured to operate within a common boundary.
PIM-SM creates unidirectional shared trees which are rooted at a common node in the network called
the rendezvous point (RP). The RP acts as the messenger between the source and the interested
hosts or routers.
There are various ways of identifying an RP within a network. It can either be statically configured per
PIM router or configured using Bootstrap Router (BSR). Within a network, the RP should always be
upstream compared to the destination hosts.
Once the RP has been identified, each interested host and/or router sends join messages to the RP
for the group that they are interested in. To reduce incoming join messages to a RP, the local network
selects one of its upstream routers as the designated router (DR). All hosts below a DR send IGMP
join messages to the DR. The DR sends only one join message to the RP on behalf of all its interested
hosts.
PIM-SM also provides the option of creating a source-based tree rooted at a router adjacent to the
tree. This provides the destination hosts with an option of switching from the shared tree to the source-
based tree if this is a shorter path between the source and the destination.
PIM topologies
This section shows diagrams of two supported PIM topologies.
The figure below shows the components for a single-VCS PIM topology.
PIM topologies
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