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| 7.3 |  | Routing Multicast Traffic |
| 7.3.10 |  | Core-based trees and PIM sparse mode |

Core-based trees (CBT) are described in RFC 2201. The CBT protocol constructs a single tree that is shared by all members of the group. Multicast traffic for the entire group is sent and received over the same tree, regardless of the source. This use of a shared tree can provide significant savings in terms of the amount of multicast state information that is stored in individual routers.

A CBT shared tree has a core router that is used to construct the tree. Routers join the tree by sending a join message to the core. When the core receives a join request, it returns an acknowledgment over the reverse path, thus forming a branch of the tree. Join messages need not travel all the way to the core before being acknowledged. If a join message encounters a router on the tree before the message reaches the core, that router terminates the join message and acknowledges it. The router that sent the join is then connected to the shared tree.

The Core-Based Tree (CBT) multicast routing protocol is truly a work-in-progress and has been so for several years. The original version, CBTv1, was superseded by CBTv2 (defined in RFC 2189) which, unfortunately, is not backward compatible with CBTv1. However, because CBTv1 was never really implemented in any production networks to speak of, backward compatibility is not an issue. CBTv2 also has not seen significant deployment to date, which is probably good, because a new draft specification for CBTv3 is already out that supersedes and is not backward compatible with CBTv2!

Protocol Independent Multicast Sparse Mode
Protocol Independent Multicast Sparse Mode is optimized for environments where there are many multipoint data streams. Sparse multicast is most useful when:

- There are few receivers in a group
- The type of traffic is intermittent

In sparse mode, each data stream goes to a relatively small number of segments in the campus network. Instead of flooding the network to determine the status of multicast members, sparse-mode PIM defines a rendezvous point. When a sender wants to send data, the sender first sends to the rendezvous point. When a receiver wants to receive data, the receiver registers with the rendezvous point. Once the data stream begins to flow from sender to rendezvous point to receiver, the routers in the path will optimize the path automatically to remove any unnecessary hops. Sparse-mode PIM assumes that no hosts want the multicast traffic unless they specifically ask for it. PIM is able to simultaneously support dense mode for some multicast groups and sparse mode for others.