Campus networks typically have a large number of subnetworks, each being their own broadcast domain. As you know, routers must connect these subnetworks so that they can be routed from one broadcast domain to the next. This is, by definition, the function of the IP protocol.
Each host on the Internet has an address that identifies the physical location of the host. Part of the address identifies the subnet on which the host resides and part identifies the individual host on that subnet. Routers periodically send routing update messages to adjacent routers, conveying the state of the network as perceived by that particular router. This data is recorded in routing tables that are then used to determine optimal transmission paths for forwarding messages across the network.
Unicast transmission involves transmission from a single source to a single destination. The transmission is directed toward a single physical location that is specified by the host address. This routing procedure is relatively straightforward because of the binding of a single address to a single host.
Routing multicast traffic is a more complex problem. A multicast address identifies a particular transmission session rather than a specific physical destination. An individual host is able to join an ongoing multicast session by using IGMP to communicate this desire to the subnet router.
Because the number of receivers for a multicast session can potentially be quite large, the source does not need to know all the relevant addresses. Instead, the network routers must somehow be able to translate multicast addresses into host addresses. The basic principal involved in multicast routing is that routers interact with each other to exchange information about neighboring routers.
Multicast routing is based upon the construction of "trees," connecting the members of the various multicast groups. The following sections discuss the types of trees and how they are constructed.
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