What Is Igmp Snooping - Dell M6220 User Configuration Manual

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.

What Is IGMP Snooping?

IGMP snooping allows the switch to snoop on IGMP exchanges between
hosts and multicast routers. The IGMP snooping feature complies with RFC
4541. When a switch "sees" an IGMP report from a host for a given multicast
address, the switch adds the host's interface/VLAN to the L2 multicast group
forwarding table and floods the report to all ports in the VLAN. When the
switch sees a leave message for the group, it removes the host interface/VLAN
from the L2 multicast group forwarding table.
IGMP snooping learns about multicast routers by listening for the following
messages:
• IGMP Membership queries
• PIMv1 hellos
• PIMv2 hellos
• DVMRP probes
Group addresses that fall into the range 224.0.0.x are never pruned by IGMP
snooping—they are always flooded to all ports in the VLAN. Note that this
flooding is based on the IP address, not the corresponding 01-00-5e-00-00-xx
MAC address.
When a multicast router is discovered, its interface is added to the interface
distribution list for all multicast groups in the VLAN. If a switch is connected
to a multicast source and no client, the switch filters the traffic from that
group to all interfaces in the VLAN. If the switch sees an IGMP join from a
host in the same VLAN, then it forwards the traffic to the host. Likewise, if
the switch sees a multicast router in the VLAN, it forwards the group to the
multicast router and does not flood in the VLAN. There is a user option to
cause the switch to flood multicast sources in the VLAN if no multicast
clients are present. This option should be enabled when deploying the switch
Configuring L2 Multicast Features
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