| 9.1 | Multicasting Overview | ||
| 9.1.1 | IGMP and CGMP: Advanced traffic management |
| Many corporations and industries are now realizing the potential
of multimedia applications. Video and voice conferencing
applications generate much of the multicast traffic today. Many Web
sites offer video streams that transmit multicast traffic. Companies
are experiencing a marked increase in multicast traffic from
internal users. Because of the proliferation of multicast
applications, network administrators are charged with installing and
configuring devices and implementing policy to manage traffic
associated with these applications. Many network designs deal
efficiently with unicast traffic, but neglect the impact of
multicast traffic within the network. Unfortunately, this can
severely impact a well-designed unicast network.
Again, it should be reiterated that it is critical in network design to take a proactive approach to controlling multicast traffic in your network. The default behavior of a switch floods multicast frames out of all ports except the originating port. This is usually not the desired effect. Cisco implements the proprietary Cisco Group Management Protocol (CGMP) to help you administer multicast traffic in your network. CGMP runs on switches and routers. CGMP works hand-in-hand with Internet Group Membership Protocol (IGMP), running on routers, to control the flooding of multicast traffic in a network. Multicast traffic originates at sources that need to distribute the same information to multiple recipients. When a source creates multicast traffic, it uses special Layer 2 and Layer 3 addresses so that routers and switches know how to route or switch the frame. By default, routers do not forward multicast traffic unless they are multicast capable and have a multicast routing protocol such as Distance Vector Multicast Routing Protocol (DVMRP) or Protocol Independent Multicast (PIM) enabled. DVMRP and PIM are inter-router protocols. Therefore, hosts and switches do not participate in messages for these protocols. A LAN switch is not a router (although a router can be incorporated, such as the route switch module [RSM]). What happens, then, to multicast traffic in a switched network? By default, a switch floods multicast traffic within a broadcast domain. This consumes bandwidth on both access links and trunk links. Depending upon the host TCP/IP stack implementation and network interface card (NIC) attributes, the multicast frame can cause a CPU interrupt. The reason a switch floods multicast traffic is that it has no entry in the bridge table for the destination address. Multicast addresses never appear as source addresses; therefore, the bridge/switch cannot dynamically learn multicast addresses; they must be manually configured. IGMP is a multicast protocol that directly affects hosts. IGMP allows hosts to inform routers that they want to receive multicast traffic for a specific multicast group address. Many switches don't understand IGMP messages (unless you have the NetFlow feature card [NFFC] installed). IGMP messages appear to a Catalyst® Switch like any other multicast frames. Cisco developed the proprietary CGMP, which enables routers to inform switches about hosts and their interest in receiving multicast traffic. This modifies the Catalyst Switch default behavior of flooding the multicast frame to all hosts in the broadcast domain. Rather than flooding the frame to all hosts, the Catalyst Switch limits the flooding scope to only those hosts in the broadcast domain that registered with the router through IGMP. If a host does not register with the router, it does not receive a copy of the multicast frame. This helps preserve access-link bandwidth.
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