Multicast Protocols User Guide
Multicast allows an IP network to support more than just the unicast model of data delivery that prevailed in the early stages of the Internet. Multicast provides an efficient method for delivering traffic flows that can be characterized as one-to-many or many-to-many.
In a multicast network, the key component is the routing device, which is able to replicate packets and is therefore multicast-capable. The routing devices in the IP multicast network, which has exactly the same topology as the unicast network it is based on, use a multicast routing protocol to build a distribution tree that connects receivers (preferred to the multimedia implications of listeners, but listeners is also used) to sources. In multicast terminology, the distribution tree is rooted at the source (the root of the distribution tree is the source). The interface on the routing device leading toward the source is the upstream interface, although the less precise terms incoming or inbound interface are used as well. To keep bandwidth use to a minimum, it is best for only one upstream interface on the routing device to receive multicast packets. The interface on the routing device leading toward the receivers is the downstream interface, although the less precise terms outgoing or outbound interface are used as well. There can be 0 to N–1 downstream interfaces on a routing device, where N is the number of logical interfaces on the routing device.