Using PIM in sparse mode is much more effective, because of the explicit requests. A multicast packet is only flooded from the sender to the first hop router. Only when a client requests data, the multicast packets are flooded out of the broadcast domain. The core control point of PIM-Sparse mode is the Rendezvous Point (RP). The RP tracks all the senders and receivers in the multicast domain.
In the figure below the loopback 0 of R1 is configured as the RP address. OSPF area 0 and PIM in sparse mode is enabled on all interfaces of all routers.
Let us check the scenario when a multicast receiver joins to a group. We assume now there are no senders.
- Client sends an IGMP membership report out on the LAN.
- IGMP join will then the converted to PIM join and sent upstream to the RP.
- A (*,G) entry will be installed on all routers in the IGP path to RP