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24-port NWay Ethernet Switch User’s Guide
Switch Management and Operating Concepts 111
Multicast Spanning Trees
A Spanning Tree is powerful and easy to implement. In this
algorithm, a subset of links are selected to define a tree
structure such that there is only one active path between any two
routers. Since this tree spans all nodes in the network, it is
called a spanning tree. Whenever a router receives a multicast
packet, it forwards the packet on all links that belong to the
spanning tree except the one on which the packet was received,
guaranteeing that the multicast packet reaches all the routers in
the network. Obviously, the only information a router needs to
keep is a Boolean variable per network interface indicating
whether the link belongs to the spanning tree or not.
The spanning tree algorithm has two drawbacks: It centralizes
all traffic on a small set of links and it des not consider group
membership it its
decisions.
Reverse Path Broadcasting (RPB)
The RPB algorithm is currently being used in the MBone is a
modification of the Spanning Tree algorithm. In RPB, instead of
building a network-wide spanning tree, an implicit spanning tree
is constructed for each source. Based on this algorithm,
whenever a router receives a multicast packet on link “A” from
source “S”, the router will check and see if the Link A belongs to
the shortest path toward S. If this is the case the packet is
forwarded on all links except L. Otherwise, the packet is
discarded. Three multicast trees from two sources of our test
network are shown below:
The RPB algorithm can be easily improved by considering the
fact that if the local router is not on the shortest path between
the source node and a neighbor, the packet will be discarded at