MNS-BB Software User Guide
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Group Management Protocol controls). In the factory default state (IGMP disabled), the switch
forwards all IGMP traffic to all ports, which can cause unnecessary bandwidth usage on ports not
belonging to multicast groups. Enabling IGMP allows the ports to detect IGMP queries, report
packets and manage IP multicast traffic through the switch.
12.2 Purpose
The purpose of IGMP Snooping is to limit multicast traffic to only those LAN segments that are
interested in receiving the messages. In normal switch operations without IGMP, IP multicast traffic
is flooded through out the whole LAN. It is flooded because a switch usually learns MAC address by
looking in to the source address field of all the frames it receives. However, since a multicast address
is never used as a source address for a packet (it has several false addresses which are not unique) and
since they do not appear in the MAC address table (because they are not real), the switch has no
method for learning them. The most efficient method to weed them out is to use IGMP Snooping.
With IGMP Snooping the switch intercepts the IGMP messages (multicast messages only) from the
host itself and updates its MAC table accordingly.
IGMP is useful in multimedia applications such as LAN TV, desktop conferencing, and collaborative
computing, where there is multipoint communication; that is, communication from one to many hosts,
or communication originating from many hosts and destined for many other hosts. In such multipoint
applications, IGMP will be configured on the hosts, and multicast traffic will be generated by one or
more servers (inside or outside of the local network). Switches in the network (that support IGMP)
can then be configured to direct the multicast traffic to only the ports where needed.
Enabling IGMP allows the ports to detect IGMP queries and report packets and manage IP multicast
traffic through the switch. If no other querier is detected, the switch will then also function as the
querier. (If you need to disable the querier feature, you can do so through the IGMP configuration
MIB. Refer to “Changing the Querier Configuration Setting”)
12.3 IGMP Operating Features
In the factory default configuration, IGMP is disabled. IGMP works only on default VLAN
(DEFAULT_VLAN; VID = 1). When you use either the CLI or the Telnet interface to enable IGMP
on the switch, the switch forwards IGMP traffic only to ports belonging to multicast groups.
¾ Auto/Blocked/Forward: You can use the console to configure individual ports to any of the following
states:
• Auto (the default): Causes the switch to interpret IGMP packets and to filter IP multicast traffic
based on the IGMP packet information for ports belonging to a multicast group. This means that
IGMP traffic will be forwarded on a specific port only if an IGMP host or multicast router is
connected to the port.
• Blocked: Causes the switch to drop all IGMP transmissions received from a specific port and to
block all outgoing IP Multicast packets for that port. This has the effect of preventing IGMP
traffic from moving through specific ports.
• Forward: Causes the switch to forward all IGMP and IP multicast transmissions through the
port.
¾ Querier: In the default state (enabled), eliminates the need for a multicast router. In most cases,
Blackbox recommends that you leave this parameter in the default “enabled” state even if you
have a multicast router performing the querier function in your multicast group.
12.4 Benefit
The IGMP Snooping feature enables the switch to monitor the flow of queries from the router and
reports from the host nodes to build its own multicast membership lists. It uses the lists to forward
multicast packets only to switch ports where there are host nodes that are members of multicast
groups. This improves switch performance and network security by further restricting the flow of
multicast packets only to those switch ports connected to host nodes.
Without IGMP Snooping, the switch would flood all multicast packets out all of its ports, except the
port on which it received the packet. Such flooding of packets can negatively impact switch and
network performance.
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