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GS700TR Smart Switch Software Administration Manual
Configuring Switching Information 3-33
v1.0, May, 2008
Configuring IGMP Snooping
Internet Group Management Protocol (IGMP) Snooping is a feature that allows a switch to
forward multicast traffic intelligently on the switch. Multicast IP traffic is traffic that is destined to
a host group. Host groups are identified by class
D IP addresses, which range from 224.0.0.0 to
239.255.255.255. Based on the IGMP query and report messages, the switch forwards traffic only
to the ports that request the multicast traffic. This prevents the switch from broadcasting the traffic
to all ports and possibly affecting network performance.
A traditional Ethernet network may be separated into different network segments to prevent
placing too many devices onto the same shared media. Bridges and switches connect these
segments. When a packet with a broadcast or multicast destination address is received, the switch
will forward a copy into each of the remaining network segments in accordance with the IEEE
MAC Bridge standard. Eventually, the packet is made accessible to all nodes connected to the
network.
This approach works well for broadcast packets that are intended to be seen or processed by all
connected nodes. In the case of multicast packets, however, this approach could lead to less
efficient use of network bandwidth, particularly when the packet is intended for only a small
number of nodes. Packets will be flooded into network segments where no node has any interest in
receiving the packet. While nodes will rarely incur any processing overhead to filter packets
addressed to un-requested group addresses, they are unable to transmit new packets onto the
shared media for the period of time that the multicast packet is flooded. The problem of wasting
bandwidth is even worse when the LAN segment is not shared, for example in Full Duplex links.
Allowing switches to snoop IGMP packets is a creative effort to solve this problem. The switch
uses the information in the IGMP packets as they are being forwarded throughout the network to
determine which segments should receive packets directed to the group address.
The Multicast folder contains links to the following features:
• “Global Configuration” on page 3-33
• “IGMP Snooping Interface Configuration” on page 3-35
Global Configuration
Use the IGMP Snooping Configuration page to configure the parameters for IGMP snooping,
which is used to build forwarding lists for multicast traffic.
To access the IGMP Snooping Configuration page: