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Configure Switching
151
Insight Managed 28-Port and 52-Port Gigabit Ethernet Smart Cloud Switches
Configure IGMP Snooping
Internet Group Management Protocol (IGMP) snooping is a feature that allows a switch to
forward multicast traffic intelligently. 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.
IGMP Snooping Overview
A traditional Ethernet network can 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 forwards a copy to 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 are flooded into network segments where no node is
receptive to the packet. While nodes rarely incur any processing overhead to filter packets
addressed to unrequested group addresses, they cannot 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 receive packets directed to the group address.
In addition to building and maintaining lists of multicast group memberships, the IGMP
snooping switch also maintains a list of multicast routers. Multicast packets are forwarded on
ports to which multicast routers are connected. With IGMP snooping, only one querier can be
active in the network. All other routers in the network are suppressed and are not detectable
by the switch. If a query is not received on an interface within a specified period, the interface
is removed from the list of interfaces to which multicast routers are attached. However, by
default, the multicast router expiration time is zero, that is, the multicast router does not
expire.
A statically configured router that is connected to an interface or VLAN on the switch is
automatically added to the list with learned multicast routers. The interface must be active or
must be both active and a member of the VLAN.
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