IGMP report messages from IP multicast clients on the LAN are forwarded to the designated router following an IGMP query message from the router directed to all clients on the LAN. The router then forwards the multicast traffic via the local switches towards the multicast clients. The normal behaviour of LAN switches is such that they flood the multicast traffic out of all access ports to ensure the correct multicast clients receive the requested traffic. This is identical to the way the Ethernet switches forward broadcast traffic, and it is not a very efficient way to deal with this traffic as it is at the very least wasteful of bandwidth.
One answer is to set the local access switches up for IGMP snooping, but this can in come circumstances impact on the performance of the switch.
CGMP allows multicast traffic to be identified by the switch and only forwarded to the specific clients who require the multicast traffic to receive it. By doing this it also reduces bandwidth usage in the LAN, thereby making the LAN segment more efficient. The hosts or clients themselves require no additional configuration and are oblivious to the fact that CGMP is running.
CGMP is configured on the designated multicast router and on the access switches that serve the clients. Up to 64 separate IP multicast group registrations can be dealt with by the router. When the router receives an IGMP report from clients, it notes the MAC Address of the client and sends a CGMP join message to the local switch, informing the switch of the multicast client’s MAC Address. The switch creates a table of MAC Address to Port number for each group and only forwards multicast traffic to the clients that are members of the specific group. In other words, the switch can now identify individual client devices by means of their MAC Address and port to which they are attached. No need to flood traffic out of every single port.
When an IGMP Leave message is sent by a client to the local router, the router creates a CGMP message that it sends to the switch informing the switch that this particular client does no longer need that particular multicast stream. The switch in turn removes the MAC Address from the group table and stops forwarding traffic to that individual client.
This method is much less CPU and Memory intensive for the switch when comparing it with the IGMP Snooping method.
CGMP is covered in our Introduction to VoIP 2 day training course and will also form part of our soon to be offered Introduction to Multicasting hands-on training course soon to be offered.
For a breakdown of our training course schedule follow http://www.nstuk.com/scheduled-courses.html