Multicast traffic originates at sources desiring 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 bridges know how to disperse the frame. By default, routers do not forward multicast traffic unless they are multicast capable and have a multicast routing protocol such as DVMRP (distance vector multicast routing protocol) or PIM (protocol independent multicast)
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 RSM). What happens, then, to multicast traffic in a switched network? By default, a switch (bridge) floods multicast traffic within a broadcast domain. This consumes bandwidth on access links and trunk links. Depending upon the host's TCP/IP stack implementation and network interface card (NIC) attributes, the multicast frame can cause a CPU interrupt. Why does a switch flood multicast traffic? A switch floods multicast traffic because 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. You can manually configure an entry with the set cam static command.
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.
Current Catalysts don't understand IGMP messages (unless you have the NetFlow Feature Card [NFFC]). IGMP messages appear to a Catalyst like any other multicast frames. Cisco developed the proprietary CGMP that enables routers to inform Catalysts about hosts and their interest in receiving multicast traffic. This modifies the Catalyst's default behavior of flooding the multicast frame to all hosts in the broadcast domain. Rather than flooding the frame to all hosts, the Catalyst 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 preserves access link bandwidth.
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