MS Series User Guide
38
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.
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