My TN Connection

Internet Group Management Protocol (IGMP) is a way to restrain broadcast or multicast traffic in a switched network. With unicast traffic, an Ethernet switch learns MAC addresses by looking into the source address field of every frame. However, with multicast packets the Ethernet switch must deal with a multicast MAC header which may or may not appear in its table. Consequently, multicast packets are copied and transmitted or “flooded” to every port. During multicast floods or transmissions devices are unable to use the network, thus wasting bandwidth at best and preventing control data from being sent at worst. The effect of multicast floods is particularly bad with full-duplex links, because the bandwidth consumed is proportional to the number of attached nodes which each invite a multicast packet.

IGMP prevents a flood of packets from congesting a network segment where a node is not interested in receiving the packets. IGMP is an integral part of IP and is used by routers to report their multicast status to nearby routers.

IGMP enabled routers are referred to as queriers. Queriers send out multicast group membership requests to look for multicast group members. The members send back membership reports. In this type of scenario, only the stations participating in the multicasts and queriers know the multicast environment.

Ethernet switches treat multicast traffic just like broadcast traffic by flooding it out all ports. To prevent this, Ethernet switches can be programmed to support IGMP Snooping. When an Ethernet switch has to look into the MAC header and snoop into the IP header before handling the packet it is called “IGMP Snooping.” IGMP Snooping allows a switch to listen in on the membership reports and queries and build multicast MAC address tables so that only the stations destined to see the multicast packets see them. The multicast packet is then directed only to those nodes listed in the router’s table of multicast addresses that are “interested” in receiving the traffic.

Because looking into two headers exacts a performance penalty some people will look to alternatives to IGMP Snooping. One of these alternatives is the manual entry of addresses in the multicast table.

In any case, the ultimate goal is to configure routers and Ethernet switches to avoid multicast floods and multicast packet leaks to nodes that have not registered an interest in receiving such packets.

View Transition Networks’ entire line of managed Ethernet switches, including those that support IGMP Snooping.

Keywords: connection, Definition, Ethernet, Ethernet Switches, IGMP Snooping, Multicast, MyTnConnection, Networks, Packets, Products, Transition Networks, Transmit, Unicast

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