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NCT480 IP DSLAM User Guide YML856 Rev3
84 www.netcomm.com.au
IGMP Snooping
The IGMP Snooping allows you to view and change the IGMP Snooping administrate status.
IP traffic can be transmitted in one of either three ways: unicast (one sender to one receiver), broadcast (one
sender to all members on the network) or multicast (one sender to a group of hosts). IGMP is a session-layer
(layer-3) protocol used to establish membership in a multicast group.
Multicast addresses are Class D IP address, from 224.0.0.0 to 239.255.255.255. These addresses are also
referred to as Group Destination Address (GDA). Each GDA address is associated with one MAC address. The
GDA MAC address is constructed by joining 01:00:5E and the last 23 bits of the GDA multicast IP address in
Hex. For example, GDA 224.1.1.1 corresponds to MAC address 01:00:5E:01:01:01
A layer-2 switch supported IGMP snooping can passively snoop on IGMP Query, Report and Leave packets
transferred between Routers/Switches and hosts to learn the IP Multicast group membership. It snoops IGMP
packets passing through it, picks out the group registration information, and configures multicasting accordingly.
Figure 6-6 IGMP Snooping Dialog
Item Description
DSLAM IGMP Snooping Status
Specified whether or not IGMP Snooping is to be enabled in the system.
(Ethernet) Port ID
This specifies the network port interface.
VPI/VCI
This specifies the virtual path, circuit identification for the PVC in the port.
IGMP Snooping Status
A Bridge Port, for which IGMP Snooping needs to be enabled or disabled.
Leave Mode
IGMP Snooping Leave message processing mode for the port.
If the mode is set to ‘Normal’, the Leave message is forwarded to the Querier and then
based on the Query received from Querier the Leave processing is triggered.
If the mode is set to ‘Fast’, the port is immediately deleted from that multicast group on
Leave message reception and then the Leave message is forwarded. The mode should
be set to ‘Fast’ for a port only if there is one host behind the port. This is because if
there are multiple hosts behind the port then it will lead to traffic disruption for other
hosts who might still be listening to that multicast group.
If mode is set to ‘FastNormal’, the Leave message is forwarded and the Leave
processing is triggered immediately without waiting for any trigger from the Querier.
‘FastNormal’ mode thus saves the delay (equal to the time taken for Leave message to
reach router and Querier processing time for it and the time taken for Query to reach
IGMP Snoop module) in Leave processing.
Table 6-4 IGMP Snooping Description