Routing loops are disastrous in multicast networks because of the risk of repeatedly replicated packets, which can overwhelm a network. One of the complexities of modern multicast routing protocols is the need to avoid routing loops, packet by packet, much more rigorously than in unicast routing protocols. Three multicast strategies—reverse-path forwarding (RPF), shortest-path tree (SPT), and administrative scoping—help prevent routing loops by defining routing paths in different ways.
The router's multicast forwarding state runs more logically based on the reverse path, from the receiver back to the root of the distribution tree. In reverse-path forwarding (RPF), every multicast packet received must pass an RPF check before it can be replicated or forwarded on any interface. When it receives a multicast packet on an interface, the router verifies that the source address in the multicast IP packet is the destinationaddress for a unicast IP packet back to the source.
If the outgoing interface found in the unicast routing table is the same interface that the multicast packet was received on, the packet passes the RPF check. Multicast packets that fail the RPF check are dropped, because the incoming interface is not on the shortest path back to the source. Routers can build and maintain separate tables for RPF purposes.
The distribution tree used for multicast is rooted at the source and is the shortest-path tree (SPT), but this path can be long if the source is at the periphery of the network. Providing a shared tree on the backbone as the distribution tree locates the multicast source more centrally in the network. Shared distribution trees with roots in the core network are created and maintained by a multicast router operating as a rendezvous point (RP), a feature of sparse mode multicast protocols.
Scoping limits the routers and interfaces that can forward a multicast packet. Multicast scoping is administrative in the sense that a range of multicast addresses is reserved for scoping purposes, as described in RFC 2365, Administratively Scoped IP Multicast. Routers at the boundary must filter multicast packets and ensure that packets do not stray beyond the established limit.