Configuring Igmp Snooping - D-Link DWS-4026 User Manual

D-Link Unified Access System Software User Manual
12/10/09
C IGMP S
ONFIGURING NOOPING
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.
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Configuring IGMP Snooping

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