TCP/IP Tutorial and Technical Overview - IBM Redbooks

To track the membership of individual groups, trees are calculated and updated
dynamically.
The algorithm maintains a reverse path table used to reach each source. This
table maps every known source network to the preferred interface used to reach
the source. When forwarding data, if the datagram arrives through the interface
used to transmit datagrams back to the source, the datagram is forwarded
through every appropriate downstream interface. Otherwise, the datagram
arrived through a sub-optimal path and is discarded. Using this process,
duplicate packets caused by network loops are filtered.
The use of RPF provides two benefits:
► RPF guarantees the fastest delivery for multicast data. In this configuration,
traffic follows the shortest path from the source to each destination.
► A different tree is computed for each source node. Packet delivery is
distributed over multiple network links. This results in more efficient use of
network resources.
6.4.2 Center-based tree algorithm
The center-based tree (CBT) algorithm describes another method to determine
optimum paths between members of a multicast group. The algorithm describes
the following steps:
1. A center point in the network is chosen. This fixed point represents the center
of the multicast group.
2. Each recipient sends a join request directed towards the center point. This is
accomplished using an IGMP membership report for that group.
3. The request is processed by all intermediate devices located between the
multicast recipient and the center point. If the router receiving the request is
already a member of the tree, it marks one more interface as belonging to the
group. If this is the first join request, the router forwards the request one step
further toward the source.
This procedure builds a delivery tree for each multicast group. The tree is
identical for all sources. Each router maintains a single tree for the entire group.
This contrasts with the process used in the RPF algorithm. The RPF algorithm
builds a tree for each sender in a multicast group.
Because there is no requirement for the source to be a member of the group,
multicast packets from a source are forwarded toward the center point until they
reach a router belonging to the tree. At this stage, the packets are forwarded
using the multicast processing of the center-based tree.
Chapter 6. IP multicast 253