Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 641)

Configuring IGMP
● IGMP overview..............................................................................................................641
● IGMP snooping overview.............................................................................................. 641
● Configuring IGMP snooping.......................................................................................... 645
IGMP overview
Internet Group Management Protocol (IGMP) is a communications protocol that allows hosts and
routers to establish group memberships, and is an integral part of IP multicast. This chapter does not
address all aspects of IGMP, but rather focuses on the snooping mechanism, as described below. For
additional commands that support basic IGMP configuration, refer to
Using additional IGMP commands
on page 647.
IGMP snooping overview
The forwarding of multicast control packets and data through a Layer 2 switch configured with VLANs is
most easily achieved by the Layer 2 forwarding of received multicast packets on all the member ports of
the VLAN interfaces. However, this simple approach is not bandwidth efficient, because only a subset of
member ports may be connected to devices interested in receiving those multicast packets. In a worst-
case scenario, the data would get forwarded to all port members of a VLAN with a large number of
member ports (for example, all 24 ports), even if only a single VLAN member is interested in receiving
the data. Such scenarios can lead to loss of throughput for a switch that gets hit by a high rate of
multicast data traffic.
Internet Group Management Protocol (IGMP) snooping is a mechanism by which a Layer 2 switch can
effectively address this issue of inefficient multicast forwarding to VLAN port members. Snooping
involves "learning" forwarding states for multicast data traffic on VLAN port members from the IGMP
control (join/leave) packets received on them. The Layer 2 switch also provides for a way to configure
forwarding states statically through the CLI.
Multicast routing and IGMP snooping
Multicast routers use IGMP snooping to learn which groups have members on each of their attached
physical networks. A multicast router keeps a list of multicast group memberships for each attached
network, and a timer for each membership.
NOTE
"Multicast group memberships" means that at least one member of a multicast group on a given
attached network is available.
There are two ways that hosts join multicast routing groups:
Network OS Administrator’s Guide 641
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