Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 270
Core-Based Tree (CBT)

The previous algorithms build spanning trees based on a source host, and multiple trees can be built from different sources. The source host is basically the root of the spanning tree and the spanning tree branches out from the source. If there are many sources, there are many roots. If there are many multicasts, each has its own multicast tree. CBT builds a single forwarding tree that is shared by all members of a group. The core of this tree (the root) is based on the core router and not the source of the multicast datagram.

CBT works on a concept that builds a backbone consisting of at least one core router. Multicast messages for a group are transmitted in this direction. Any host that wishes to receive multicast information for a specific group transmits a Join message and transmits it towards the core backbone. Each source must be configured with at least one IP address of the core routers. The core consists of at least one router that acts as the core. There can be multiple routers acting as core routers and, if so, the links that connect these core routers become the core backbone.

If there are multiple groups in the network, then multiple trees may be built. It is not the concept of one multicast tree for all groups. However, there is only one multicast tree for each group.

After issuing the Join message, each intermediate router marks the interface and multicast group and then forwards the message towards the core. In doing this, the router is able to forward multicast data towards the core for that group. When the core routers receive this data, they will multicast the data back out all ports, except the one on which it received the data.

Core-Based Tree (CBT)

This algorithm may simply stay an architecture for which other protocols will be developed. To become a protocol, issues of dynamic selection of the core backbone and management must be settled. The most notable protocol using it is the Protocol Independent Multicast (PIM) Sparse Mode (discussed later). There are advantages and disadvantages of the algorithm: Since each group is based on a single tree rooted at the core, state information on the router is easier to maintain, thereby requiring fewer resources on the router. Information that must be passed between routers to maintain these states is also less, resulting in better efficiency of the bandwidth. However, since all individual data and control messages travel towards a specific core, congestion may be inevitable.