ELTEX MA4000-PX, Руководство по эксплуатации и обслуживанию

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User Manual, Part III 73
Subscriber access/aggregation node МA4000-РХ
23 MULTICAST CONFIGURATION
23.1 Introduction
The Chapter describes peculiarities of IPTV service configuration.
Internet Group Management Protocol (IGMP) and MLD (Multicast Listener Discovery) are used in
hosts and routers for multicasting support. It provides all systems of a physical network with relevant
information: which hosts are included in groups and which group corresponds to a host.
IGMP snooping is a technique that allows network devices of the channel level (switches) to
snoop IGMP requests from hosts to a group router in order to decide whether group traffic transmission
to the corresponding interfaces should be started or stopped. When a switch snoops a host's IGMP
request for connection to a multicast group, it adds the port the host is connected to into the group (for
group traffic retranslation). And vice versa, having snooped a "leave_group" request, the switch
removes the corresponding port from the group.
Fig. 27
—
IGMP Snooping Is Disabled
Figure 27 shows multicasting of IGMP traffic regardless of whether an end host needs the traffic
or not.
When IGMP snooping becomes enabled, the multicasting situation changes as follows: the switch
will analyse all IGMP packets between connected devices and the routers the multicast traffic comes
from. When the switch receives a consumer's IGMP request for connection to a multicast group, it adds
the port the consumer is connected to into the group. And vice versa, having received a request for
leaving a group, the switch removes the corresponding port from the group.