CGMP is a simple protocol, the routers are the only devices that are producing CGMP messages. The switches only listen to these messages and act upon it. CGMP uses a well-known destination MAC address (0100.0cdd.dddd) for all its messages. When switches receive frames with this destination address, they flood it on all their interfaces which so all switches in the network will receive CGMP messages.
Within a CGMP message, the two most important items are:
* Group Destination Address (GDA)
* Unicast Source Address (USA)
The group destination address is the multicast group MAC address, the unicast source address is the MAC address of the host (receiver).
