Multicast SPT and RPT

When implementing PIM sparse mode, multicast routers use two constructs known as the root path tree (RPT) and the shortest path tree (SPT).

The RPT is a result of multicast routers joining the RP by sending PIM join messages upstream to reach the RP of the topology. This is also known as a shared distribution tree. The RP is the root of the tree which decides where to forward multicast traffic. Each multicast group may have different sources and receivers, so we may have multiple RPTs on the network.

The SPT, as its name suggests, is the shortest route between the multicast source, and a particular destination. The SPT is specific to a particular source/destination pair. It is a more efficient routing path for traffic but requires more memory in the multicast routers to maintain since there are many different SPTs since there are many different source/destination pairs.

By default, multicast traffic will use the RPT for the first packet, but if there is a better route, it will then switch to SPT for a more optimal traffic path. This switch from the RPT to the SPT can be delayed using the ip pim spt-threshold command.

When PIM dense mode is used, the RPT is primarily used, with each source having a separate distribution tree.