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Multicast is, at best, difficult to deploy in large scale networks—PIM sparse and BIDIR are both complex, adding large amounts of state to intermediate devices. In the worst case, there is no apparent way to deploy any existing version of PIM, such as large-scale spine and leaf networks (variations on the venerable Clos fabric). BEIR, described in RFC8279, aims to solve the per-device state of traditional multicast.

In this network, assume A has some packet that needs to be delivered to T, V, and X. A could generate three packets, each one addressed to one of the destinations—but replicating the packet at A is wastes network resources on the A->B link, at least. Using PIM, these three destinations could be placed in a multicast group (a multicast address can be created that describes T, V, and X as a single destination). After this, a reverse shortest path tree can be calculated from each of the destinations in the group towards the source, A, and the correct forwarding state (the outgoing interface list) be installed at each of the routers in the network (or at least along the correct paths). This, however, adds a lot of state to the network.
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