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.

Following is a brief description of terms and concepts used to describe the IGMP Proxy:

Also referred to as the proxy interface. A proxy interface is an interface on which IGMP proxy service is configured. It is in the direction toward the root of the multicast forwarding tree. An upstream interface acts as a host running IGMP; therefore, it is also called host interface.

An interface that is running IGMP and in the direction contrary to the root of the multicast forwarding tree. A downstream interface acts as a router running IGMP; therefore, it is also called router interface.

State of the associated group address and interface.

This section provides the configuration steps for configuring IGMP Proxy and example for a relevant scenario.

Unicast routing protocol should be configured in the PIM domain.

In this example, Routers 2 and 3 are running PIM and Router1 is the IGMP Proxying router.

Enable IP multicast routing on all of the PIM routers inside the PIM domain:

Enable IGMP proxy service on the interface in which the interface is in the direction toward the root of the multicast forwarding tree. In this example eth1 is the upstream interface which acts as an IGMP host.

Enable IGMP mrouter proxy on the interface in which the interface is in the direction contrary to the root of the multicast forwarding tree. In this example eth2 is the downstream interface which is connected to receiver.

Here is the same configuration for IGMP Proxying router.

The following output displays the IGMP Proxy interface information.

The following output displays the IGMP proxy information.

The following output displays the IGMP proxy group membership information.

Enable IGMP proxy unsolicited report interval on the upstream interface. The proxy group membership reports are forwarded to the upstream router in this unsolicited report interval time.

Here is the same configuration for IGMP Proxying router.

The following output displays the IGMP proxy unsolicited report interval information.

The following output displays the IGMP proxy group membership information when the proxy unsolicited report interval is configured to specific value.

The show ip mroute command displays the IP multicast routing table.