This section describes the IGMP On-Demand function and the difference from the standard IGMP protocol after IGMP On-Demand is enabled.
By sending IGMP Query messages to the connected host and receiving IGMP Report messages and Leave messages from the host, a multicast device can know which multicast group contains receivers on the relevant network segment. The device connected to the multicast device, however, may be not a host, but an access device that is enabled with IGMP proxy.
To reduce packet exchange between the multicast device and the access device, you can perform optimization. After converging the IGMP Report/Leave status of the IGMP multicast group, the access device reports the IGMP Report/Leave status to the multicast device only if the status of the IGMP multicast group is changed. In other words, the access device sends IGMP Report messages to the multicast device only if the first member joins the multicast group, and sends the IGMP Leave message to the multicast device only if the last member leaves the multicast group. This is called IGMP On-Demand.
The multicast device enabled with IGMP On-Demand does not send the IGMP Query message initiatively to identify whether the IGMP multicast group contains receivers on the network segment, but maintains the IGMP multicast group by receiving the Report/Leave status of the multicast group converged by its connected access device (IGMP proxy).
IGMP On-Demand is applied to IGMPv2 and IGMPv3 only. After a multicast device is enabled with IGMP On-Demand, the multicast device implements IGMP different from the standard one, as shown follows:
The multicast device does not send IGMP Query messages initiatively.
After the multicast device receives the IGMP Report message, the multicast device creates the entry about the multicast group and multicast source, and the entry never expires.
The multicast device deletes the relevant entries only if it receives the IGMP Leave message.