3Com CoreBuilder 3500, Implementation Manual, Page: 344 / 592

344 C
HAPTER
13: IP M
ULTICAST
R
OUTING
The term spanning tree applies to any loopless graph that spans
intelligent nodes. The DVMRP spanning tree structure provides only one
active path to connect any two multicast routers in the network. This
approach provides a logical, efficient path to reach group members and
prevents multicast storms from decreasing network performance.
The Spanning Tree Algorithm that is specified in the IEEE 802.1D MAC
Bridges base standard is a different implementation of the spanning tree
concept; it is not used with IP multicast.
Managing the
Spanning Tree
RPM uses three main techniques to dynamically adjust the shape of an IP
multicast spanning tree: broadcasting, pruning, and grafting. These
techniques balance the goal of an efficient delivery path with the goal of
effective service for all potential group members. Figure 69 shows the
broadcasting, pruning, and grafting processes.
Figure 69
RPM Techniques for Managing the Multicast Spanning Tree
Broadcasting
Source
Pruning
Potential
hosts
Grafting
Source
= Multicast router
= Pruned branch
= Active branch
= Graft
= LAN without
multicast hosts
= Leaf with
multicast hosts
Source