CGMP is a proprietary Cisco protocol and Layer 2 multicasting feature that works in conjunction with IGMP in order to make the forwarding of multicast frames more efficient. Because the protocol is Cisco specific, it only works on Cisco routers and switches. Basically, CGMP is a protocol used by a Cisco router to inform a Catalyst switch of the specific host that wishes to receive a multicast. With information about the specific hosts that wish to receive a multicast, the switch can then filter the multicast traffic, ensuring that only the hosts that require it actually receive it.
You should recall that switches make forwarding decisions based on MAC addresses rather than IP addresses. However, when a host wishes to join a multicast group for the purpose of receiving a transmission, it sends a membership report to the local router. When the router forwards the multicast onto the network, it cannot use the destination MAC address of the single host, because if this were the case, all other hosts who need the multicast would reject it. Instead, the router uses a specially created MAC address that is a variation on the multicast IP address. You don’t to know the technical details of how this happens for the CCDA, but it is sufficient to say that the MAC address used unique identifies the multicast stream. When switches receive these frames, the MAC address shows them as being a multicast, and the frames will be forwarded out all ports. However, when CGMP is enabled on both switches and the local router, a different process takes place.
When the router receives the original IGMP message from a host requesting a multicast, this message includes the host’s source IP address and MAC address. Based on this information, the router is able to send a CGMP message to switches, letting them know both the destination MAC address of the host, and the destination MAC address of the multicast. With this information, a switch is able to dynamically add another entry to its MAC address table, specifying that the port should receive traffic for both MAC addresses. Ultimately, this ensures that only the hosts that require the multicast stream are forwarded it. This eliminates the need for the switch to forward the multicast to all ports.
