User guide

Old IGMP Snooping Functionality IGMP snooping
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iMG/RG Software Reference Manual (IGMP)
3.1.3 Old IGMP Snooping Functionality
The following sections describe the IGMP snooping functionality for iMG models belonging to group Fiber-A,
Fiber-C, and ADSL-A.
3.1.3.1 Multicast router port discovery
IGMP snooping is activated using the IGMP SNOOPING ENABLE command.
The system listens for IGMP Membership General Query packets sent to the address 01-00-5e-00-00-01 and
records the port(s) where any such message has been received.
In this way the Residential Gateway knows where multicast routers are located in order to forward report and
leave messages only to the correct port(s).
Note that even if multiple VLANs can be present in the system, the IGMP snooping feature can be turned on
only on one VLAN at time.
3.1.3.2 Snoop-Only Operation Mode
3.1.3.2.1 Joining a Multicast Group
The system listens for unsolicited IGMP Report messages that hosts send to join a multicast group and records
the port where each message has been received. What happens next depends on the circumstances in which
the packet is received. To understand this, let us consider two possible scenarios:
• First Scenario:
Host A is the first host in an Ethernet segment to join a group.
Host A sends an unsolicited IGMP Membership report.
The Residential Gateway intercepts the IGMP membership report sent Host A and creates a multicast entry for
the group that host A was requesting. It then links this entry to the port on which it has received the report.
It also sets, for this port and this multicast group , a local Timeout timer to the Timeout Interval value This
timer is used to refresh the multicast membership table periodically.
The system then forwards the IGMP report on to the multicast router. In this way the router will also receive
the IGMP report and will update its multicast routing table accordingly. If no Multicast router has been detected,
then it does nothing.
Immediately multicast traffic for the requested group address is forwarded only to the port where the report
from Host A has been received.
• Second Scenario:
Another host B, on the same Ethernet segment as host A joins the same multicast group as host A.