Chapter 13. Multicast and Broadcast Services

This chapter covers the following key topics:

• CGMP/IGMP: Advanced Traffic Management Covers the protocol operations, message formats, and options in addition to how CGMP and IGMP work in routed and switched environments. This section also provides details on the steps necessary to make these protocols active in your Catalysts and routers.

• IGMP Snooping: Advanced Traffic Management Covers another method of controlling multicasts in a Catalyst network.

• Broadcast Suppression: Advanced Traffic Management Explains broadcast suppression methods in a Catalyst network.

Many corporations and industries now realize the potential of multimedia applications. Video and voice conference applications generate much of the multicast traffic today. Many Web sites offer video streams that transmit multicast traffic. Organizations, therefore, are experiencing an increase in multicast traffic from internal users and from Web downloads. If you do not see much multicast traffic in your system today, be prepared because you soon will. As a result of the proliferation of multicast applications, network administrators have to plan and deal with multicast traffic loads in more frequency than in the past. Many network plans deal efficiently with unicast traffic, but neglect the impact of multicast services in the network. Unfortunately, this severely impacts the well-thought-out unicast network.

With multicast traffic present in networks, wisdom dictates that you should proactively control the distribution of multicast throughout your network. The default behavior of a switch floods the frame everywhere in the broadcast domain. This is usually not necessary, but happens because of bridge rules. Cisco implements the proprietary Cisco Group Management Protocol (CGMP) to help you administer multicast in your network. CGMP works in a Catalyst and router along with Internet Group Membership Protocol (IGMP) in a router to control the flooding of multicast traffic in a network. The Catalyst 5000, 6000, 4000, 2948G, 2926G, and 2926 models support multicast suppression through CGMP.

Although this chapter does not intend to be a definitive source for multicast issues, a brief overview of multicast usage, addressing, IGMP operations (both version 1 and version 2), and CGMP is germane to the Catalyst. The following sections, therefore, provide background information on these topics.