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devices. With IGMP proxying 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 is a host but no longer a PIM neighbor to the upstream
device.
Figure 36 Network diagram
As shown in Figure 36, the following types of interfaces are defined on an IGMP proxy device:
• Upstream interface—Also called the "proxy interface". A proxy interface is an interface on which
IGMP proxying is configured. It is in the direction toward the root of the multicast forwarding tree.
An upstream interface acts as a host that is running IGMP. Therefore, it is also called the "host
interface".
• Downstream interface—An interface that is running IGMP and is not in the direction toward the
root of the multicast forwarding tree. A downstream interface acts as a router that is running IGMP.
Therefore, it is also called the "router interface".
A device with IGMP proxying 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.
Query from Router A
Report from Router B
Ethernet
Router interface
Host interface
Proxy & Querier
Router B
Querier
Router A
Host B
Receiver
Host A
Receiver
Host C
Query from Router B
Report from Host
PIM domain
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