3Com S7902E, Configuration Manual

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1-6
Figure 1-3 Network diagram for IGMP SSM mapping
SSM
IGMPv1 report
IGMPv2 report
IGMPv3 report
Router A
Querier
Host A (IGMPv1) Host B (IGMPv2) Host C (IGMPv3)
Receiver Receiver Receiver
As shown in Figure 1-3 , on an SSM network, Host A, Host B and Host C are running IGMPv1, IGMPv2
and IGMPv3 respectively. To provide SSM service for all the hosts while it is infeasible to run IGMPv3 on
Host A and Host B, you need to configure the IGMP SSM mapping feature on Router A.
With the IGMP SSM mapping feature configured, when Router A receives an IGMPv1 or IGMPv2 report,
it checks the multicast group address G carried in the message:
z
If G is not in the SSM group range, Router A cannot provide the SSM service but the ASM service.
z
If G is in the SSM group range but no IGMP SSM mappings corresponding to the multicast group G
have been configured on Router A, Router A cannot provide SSM service and drops the message.
z
If G is in the SSM group range and the IGMP SSM mappings have been configured on Router A for
multicast group G, Router A translates the (*, G) information in the IGMP report into (G, INCLUDE,
(S1, S2...)) information based on the configured IGMP SSM mappings and provides SSM service
accordingly.
z
The IGMP SSM mapping feature does not process IGMPv3 reports.
z
For more information about the SSM group range, refer to PIM Configuration in the IP Multicast
Volume .
IGMP Proxying
In some simple tree-shaped topologies, it is not necessary to configure complex multicast routing
protocols, such as PIM, on the boundary devices. Instead, you can configure IGMP proxying on these
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.