The menu Interfaces & Routing > Multicast Routing (PIM-SM) enables you to configure Protocol Independent Multicast Sparse Mode (PIM-SM) for use on your network. PIM is a protocol to dynamically route multicast packets in networks. Multicast is a technique to deliver packets that are to be received by more than one client efficiently using as little traffic as possible. Normally, packets for more than one client are simply copied and sent to every client individually, multiplying the consumed bandwidth by the number of users. Thus servers which have a lot of clients requesting the same packets at the same time, like e.g. servers for streaming content, need a lot of bandwidth.
Multicast, in contrast, saves bandwidth by sending packets only once over each link of the network. To achieve this, multicast includes adequately configured routers in the decision when to create copies on the way from the server (sender) to the client (receiver). The routers use PIM-SM to keep track of active multicast receiver(s) and use this information to configure routing.
A rough scheme of PIM-SM communication is as follows: A sender starts transmitting its multicast data. The multicast router for the sender registers via PIM-SM with the RP router which in turn sends a join message to the sender's router. Multicast packets now flow from the sender to the RP router. A receiver registers itself via an IGMP broadcast for this multicast group at its local PIM-SM router. This router sends a join request for the receiver towards the RP router, which then in turn forwards multicast traffic to the receiver.