Multicast distribution trees control the path that multicast packets take to the destination hosts. The two types of distribution trees are source and shared. With source trees, the tree roots from the source of the multicast group and then expands throughout the network in spanning-tree fashion to the destination hosts. Source trees are also called shortest-path trees (SPTs) because they create paths without having to go through a rendezvous point (RP). The drawback is that all routers through the path must use memory resources to maintain a list of all multicast groups. PIM-DM uses a source-based tree.

Shared trees create the distribution tree’s root somewhere between the network’s source and receivers. The root is called the RP. The tree is created from the RP in spanning-tree fashion with no loops. Sources initially send their multicast packets to the RP, which, in turn, forwards data to the member of the group in the shared tree.

The advantage of shared trees is that they reduce the memory requirements of routers in the multicast network. The drawback is that initially the multicast packets might not take the best paths to the receivers because they need to pass through the RP. After the data stream begins to flow from sender to RP to receiver, the routers in the path optimize the path automatically to remove any unnecessary hops. The RP function consumes significant memory on the assigned router. PIM-SM uses an RP.

PIM

Protocol Independent Multicast (PIM) comes in two flavors: Protocol Independent Multicast–Sparse Mode (PIM-SM) and Protocol Independent Multicast–Dense Mode (PIM-DM). PIM-SM uses shared trees and RPs to reach widely dispersed group members with reasonable protocol bandwidth efficiency. PIM-DM uses source trees and reverse path forwarding (RPF) to reach relatively close group members with reasonable processor and memory efficiency in the network devices of the distribution trees.

With RPF, received multicast packets are forwarded out all other interfaces, allowing the data stream to reach all segments. If no hosts are members of a multicast group on any of the router’s attached or downstream subnets, the router sends a prune message up the distribution tree (the reverse path) to tell the upstream router not to send packets for the multicast group. So, the analogy for PIM-DM is the push method for sending junk mail, and the intermediate router must tell upstream devices to stop sending it.

PIM-SM

Protocol Independent Multicast–Sparse Mode (PIM-SM), which is defined in RFC 7761, assumes that no hosts want to receive multicast traffic unless it is specifically requested. The RP gathers the information from senders and makes the information available to receivers. Routers with receivers have to register with the RP. The end-host receivers request multicast group membership using IGMP with their local last-hop routers. The routers serving the end systems then register as traffic receivers with the RPs for the specified group in the multicast network. Senders register with the RP via the first-hop router.

The shared tree for each multicast group is rooted at the RP multicast router. Different multicast groups can use separate RPs within a PIM domain.