/mcpbandwidth if many multicast servers are sending streams to the segment. Before You Begin Conventions For more information on document conventions, see the Cisco Technical Tips Conventions. Prerequisites There are no specific prerequisites for this document. Components Used This document is not restricted to specific software and hardware versions. Background Information Multicast traffic becomes flooded because a switch usually learns MAC addresses by looking into the source address field of all the frames it receives. A multicast MAC address is never used as source address for a packet. Such addresses do not appear in the MAC address table, and the switch has no method for learning them. The first solution to this issue is to configure static MAC addresses for each group and each client. This solution works well, however, it is neither scalable nor dynamic. You use this solution on a Catalyst 4000, 5000, or 6000 switch by issuing one of the following commands: set cam static <multicast_mac> <mod/port> G set cam permanent <multicast_mac> <mod/port> G These two commands have the same effect, except that the static entries disappear at reboot, and permanent entries do not. The second solution is to use CGMP, which is a Cisco proprietary protocol that runs between the multicast router and the switch. CGMP enables the Cisco multicast router to understand IGMP messages sent by hosts, and informs the switch about the information contained in the IGMP packet. The last (and most efficient) solution is to use IGMP snooping. With IGMP snooping, the switch intercepts IGMP messages from the host itself and updates its MAC table accordingly. Advanced hardware is required to support IGMP snooping. CGMP configurations given in this document are for Catalyst 4000 and 5000 switches running CatOS (CGMP is not supported on Catalyst 6000 switches), and IGMP snooping configurations are for Catalyst 5000 and 6000 switches running CatOS. The following section briefly describes a multicast address, explains the functionality of IGMP, and provides additional detail on CGMP and IGMP snooping. Multicast Address