Figure 6 illustrates an IGMP proxy. An IGMP proxy performs functions of both an IGMP router and an IGMP client. When an IGMP host issues a join message, the IGMP proxy receives the message and adds the interface to its outgoing interface table for a specific multicast group. The proxy uses a general membership query timer and state to send general queries downstream to all multicast-enabled interfaces. When the IGMP proxy receives a leave message, the proxy issues a group-specific query. If no hosts respond to the query within a configured response time interval, the proxy removes the interface from the outgoing interface table.
Figure 6: IGMP Proxy
A device that functions as an IGMP proxy participates in every IGMP flow. This level of participation requires much more processing power and memory allocation from the DSLAM, but it can save upstream bandwidth.
Because a multicast router treats any IGMP proxy that it interacts with as an IGMP client, the multicast router tracks one device (the DSLAM) joining and leaving multicast groups. As a result, the multicast router receives no information regarding subscribers on the other side of the IGMP proxy.
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