In some simple tree topologies, it is not necessary to configure complex multicast routing protocols, such as PIM, on the boundary devices. It is sufficient to learn and proxy the group membership information and simply forward multicast packets based upon that information. Using IGMP forwarding (RFC 4605) to replicate multicast traffic on devices such as the edge boxes can greatly simplify the design and implementation of those devices. By not supporting more complicated multicast routing protocol such as Protocol Independent Multicast (PIM), it reduces not only the cost of the devices but also the operational overhead. Another advantage is that it makes the proxy devices independent of the multicast routing protocol used by the core network routers.

IGMP proxy can be used in such topologies instead of PIM. With IGMP proxy configured, the device serves as a proxy for the downstream hosts to send IGMP messages, maintain group memberships, and implement multicast forwarding based on the memberships. In this case, each boundary device configured with IGMP proxying is a host but no longer a PIM neighbor to the upstream device.

A device with IGMP proxy configured maintains a group membership database, which stores the group memberships on all the downstream interfaces. Each entry comprises the multicast address, filter mode, and source list. Such an entry is a collection of members in the same multicast group on each downstream interface.

A proxy device performs host functions on the upstream interface based on the database. It responds to queries according to the information in the database or sends join/leave messages when the database changes. On the other hand, the proxy device performs router functions on the downstream interfaces by participating in the querier election, sending queries, and maintaining memberships based on the reports.