PIM is IP routing protocol-independent and can leverage whichever unicast
routing protocols are used to populate the unicast routing table. PIM dense
mode operation is based on the assumption that the multicast group members are
densely distributed throughout the network and that bandwidth is plentiful,
meaning that almost all hosts on the network belong to the group. PIM
dense-mode multicast routing protocol relies on periodic flooding of the
network with multicast traffic to set up and maintain the distribution
tree.
PIM dense mode works best when there are numerous members belonging
to each multimedia group. PIM floods the multimedia packet out to all routers
in the network and then prunes routers that do not support members of that
particular multicast group.

PIM dense
mode is most useful under the following circumstances:
- Senders and receivers are in close proximity to one another.
- There are few senders and many receivers.
- The volume of multicast traffic is high.
The stream of multicast traffic is constant.
PIM Sparse
Mode
The second approach to multicast routing is based on the
assumption that the multicast group members are sparsely distributed throughout
the network and bandwidth is not necessarily widely available.

It
is important to note that sparse mode does not imply that the group has few
members, just that they are widely dispersed. In this case, flooding would
unnecessarily waste network resources. Sparse-mode multicast routing protocols
rely on more selective techniques to set up and maintain multicast trees.
Sparse-mode protocols begin with an empty distribution tree and add branches
only as the result of explicit requests to join the distribution.
Sparse-mode PIM is optimized for environments where there are many
multipoint data streams. Sparse multicast is most useful when:
- There are few receivers in a group.
- The type of traffic is intermittent.
In sparse mode, each data stream goes to a relatively small number of
segments in the campus network. Instead of flooding the network to determine
the status of multicast members, sparse-mode PIM defines a rendezvous point.
When a source begins to generate a flow, it is directed to a rendezvous point.
When a router determines that it has receivers out its interfaces, it registers
with the rendezvous point. The routers in the path will optimize the path
automatically to remove any unnecessary hops. Sparse-mode PIM assumes that no
hosts want the multicast traffic unless they specifically request it.
PIM is able to simultaneously support dense mode for some multicast groups
and sparse mode for others. Cisco has implemented an alternative to choosing
just dense mode or just sparse mode on a router interface. PIM sparse-dense
mode allows the network to determine which IP Multicast groups should use
sparse mode and which groups should use dense mode. PIM sparse mode and
sparse-dense mode require the use of a rendezvous point.