Protocol Independent Multicast sparse mode (PIM-SM) requires a physical rendezvous point (RP). An RP is a well-connected, centrally located router that is responsible for keeping track of multicast group membership information. When a host wants to join a multicast group, it sends an Internet Group Management Protocol (IGMP) membership report message to its local router. The local router adds the interface to the multicast tree and forwards the message to the RP. This process creates a branch of the multicast tree from the host to the RP. A branch is not pruned until the group member leaves the group.
Bidirectional PIM (bidir-PIM) can use a physical RP, but the RP does not have to be a physical device. Instead, bidir-PIM can use a phantom RP, which is an address that is used as the RP address but is not assigned to a physical device. A physical RP is not required with bidir-PIM, because bidir-PIM designated forwarders (DFs) can forward traffic up the shared tree directly to multicast receivers.
PIM dense mode (PIM-DM) does not require an RP to keep track of multicast group membership information. Instead, PIM-DM routers assume that all interfaces contain group members, so they periodically flood multicast traffic out all available interfaces, which causes a traffic spike. Each router in the network determines whether any hosts are interested in receiving the multicast traffic. If so, the router forwards the multicast traffic. If not, the router sends a prune message back to the multicast source and that branch of the multicast tree is pruned for a short period of time.
PIM sparse-dense mode (PIM-SDM) does not require an RP. PIM-SDM uses a combination of sparse mode and dense mode. The mode is determined on a per group basis. PIM-SDM routers use sparse mode if an RP exists for a multicast group and use dense mode if no RP exists for a multicast group.
PIM Source-Specific Multicast (PIM-SSM) does not require an RP. PIM-SSM is best suited for one-to-many applications, which are also called broadcast applications.
When PIMSSM is used, a multicast host can specify the source addresses from which it will accept multicast traffic. Like PIM-DM, PIM-SSM uses source based distribution trees, which are built from the multicast source to the multicast receivers.