The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

Chapter 10. Multicast Pruning

The default behavior for a switch is to flood all multicast traffic. When a switch receives a frame destined for a multicast address, it will normally forward the frame out all ports except the port on which it arrived. This ensures that all parties that might be interested in hearing that multicast frame do, in fact, hear it. If the level of multicast traffic is low, flooding provides for proper application operation without significantly increasing the traffic burden on the catenet. However, if there are applications using multicast in a traffic-intensive manner, the inefficiency of flooding becomes problematic. Flooding increases the traffic load on all active paths in the spanning tree, regardless of whether any given link is really needed to carry the traffic to those stations interested in receiving it.

In this chapter we look at mechanisms for restricting multicast traffic propagation within the catenet to only those links needed to ensure proper end-station application behavior.