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| 7.3 |  | Routing Multicast Traffic |
| 7.3.2 |  | Distribution trees |

To deliver multicast packets to all receivers, designated routers construct a tree that connects all members of an IP multicast group. A distribution tree specifies a unique forwarding path between the subnet of the source and each subnet containing members of the multicast group.

A distribution tree has just enough connectivity so that there is only one, loop-free path between every pair of routers. Because each router knows which of its lines belong to the tree, the router can copy an incoming multicast datagram onto all the outgoing branches. This action generates the minimum needed number of datagram copies. Because messages are replicated only when the tree branches, the number of copies of the messages transmitted through the network is minimized.

Because multicast groups are dynamic, with members joining or leaving a group at any time, the distribution tree must be dynamically updated. Branches that contain new members must be added. Branches in which no listeners exist must be discarded, or pruned.

There are two basic tree construction techniques: source-specific trees and shared, or center-specific trees.

Source Distribution Tree
The simplest form of a multicast distribution tree is a source tree whose root is the source of the multicast traffic and whose branches form a spanning tree through the network to the receivers. Because this tree uses the shortest path through the network, it also is referred to frequently as a shortest-path tree (SPT).

Figure shows an example of an SPT for Group 224.1.1.1 rooted at the source, Host A, and connecting two receivers, Hosts B and C.

The special notation of (S,G), pronounced "S comma G", enumerates an SPT where S is the IP address of the source and G is the multicast group address. Using this special notation, the SPT for the example in Figure would be written as (192.1.1.1, 224.1.1.1).

Notice that this notation implies that a separate SPT exists for every individual source originating a stream for a mulitcast group. Therefore, if Host B is also sending traffic to Group 224.1.1.1 and Hosts A and C are receivers, then a separate (S,G) SPT would exist with a notation of (192.2.2.2, 224.1.1.1).

Shared Distribution Tree
Unlike source trees that have their roots at the source, share trees use a single common root placed at some chosen point in the network. Depending on the multicast routing protocol, this root is often called a rendezvous point (RP) or core, which lends itself to other common names of shared trees: RP trees (RPT) or core-based trees (CBTs).

Figure shows a shared tree for Group 224.2.2.2 with the root located at Router D. When using a shared tree, sources must send their traffic to the root for the traffic to reach all receivers.

In this example, multicast group traffic from source Host A and Host D travels to the root (Router D) and then down the shared tree to two receivers, Hosts B and C. Because all sources in the multicast group use a common shared tree, a wildcard notation written as (*,G), pronounced "star comma G", represents the tree. In this case, * means all sources, and the G represents the multicast group. Therefore, the shared tree shown in Figure would be written (*, 224.2.2.2).