Multicast terms
The following terms are commonly used in discussing multicast-capable devices. These terms are used throughout this chapter:
Node: A device.
Root Node: The node that initiates the tree building process. It is also the device that sends the multicast packets down the multicast delivery tree.
Upstream: The direction from which a device receives multicast data packets. An upstream device is a node that sends multicast packets.
Downstream: The direction to which a device forwards multicast data packets. A downstream device is a node that receives multicast packets from upstream transmissions.
Group Presence: A multicast group has been learned from one of the directly connected interfaces. Members of the multicast group are present on the device.
Intermediate nodes: Devices that are in the path between source devices and leaf devices.
Leaf nodes: Devices that do not have any downstream devices.
Multicast Tree: A unique tree is built for each source group (S,G) pair. A multicast tree is comprised of a root node and one or more nodes that are leaf or intermediate nodes.