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When the IGMP SSM mapping feature is configured, Router A checks the multicast group address G in
the received IGMPv1 or IGMPv2 report, and does the following:
• If G is not in the SSM group range, Router A provides the ASM service.
• If G is in the SSM group range but does not have relevant IGMP SSM mappings, Router A drops the
message.
• If G is in the SSM group range and has relevant IGMP SSM mappings, Router A translates the (*,
G) information in the IGMP report into (G, INCLUDE, (S1, S2...)) information to provide the SSM
service.
NOTE:
The IGMP SSM mapping feature does not process IGMPv3 reports.
For more information about SSM group ranges, see "Configuring PIM."
IGMP proxying
In a simple tree-shaped topology, it is not necessary to run multicast routing protocols, such as PIM, on
edge devices. Instead, you can configure IGMP proxying on these devices. With IGMP proxying
configured, the edge device serves as an IGMP proxy for the downstream hosts. It sends IGMP messages,
maintains group memberships, and implements multicast forwarding based on the memberships. In this
case, the IGMP proxy device is a host but no longer a PIM neighbor to the upstream device.
Figure 25 IGMP proxying
As shown in Figure 25, an IGMP proxy device has the following types of interfaces:
• 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."
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