PIM sparse mode was designed for networks where multicast receivers are sparsely located across subnets on the network. PIM spare mode also works quite well in dense multicast receiver environments too.
PIM sparse mode uses the unicast routing table to carry out reverse path forwarding checks, and can utilise any routing protocol that populates the unicast routing table – it is protocol independent.
Sparse mode utilises an explicit join model where the host receivers need to send an IGMP join to their local connected router, known as the last hop router.
This join causes the last hop router to send a PIM join upwards towards the rendezvous point router on the network. If the last hop router knows the rendezvous point of the multicast group, it can send the join request towards that router. If the last hop router does not know the IP address of the rendezvous point, the join request will travel hop by hop, building a branch towards the rendezvous point to discover it
The multicast forwarding state is created through these explicit joins. It works in the opposite way of a flood and join where routers need to explicitly state that they wish to receive a multicast stream.
When a multicast source begins sending traffic, the source needs to send that traffic towards the rendezvous point router. The rendezvous point router sends traffic onwards towards the next router where a receiver is located, working its way down the tree.