How Igmp Works - Lenovo RackSwitch G8264 Application Manual

How IGMP Works

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When IGMP is not configured, switches forward multicast traffic through all ports,
increasing network load. When IGMPv2 is configured on a switch, multicast traffic
flows as follows:
 A server sends multicast traffic to a multicast group.
 The Mrouter sends Membership Queries to the switch, which forwards them to all
ports in a given VLAN.
 Hosts respond with Membership Reports if they want to join a group. The switch
forwards these reports to the Mrouter.
Any new IGMP report which causes the IGMP entries limit to be exceeded will
still be sent to the multicast router(s).
 The switch forwards multicast traffic only to hosts that have joined a group.
The Mrouter periodically sends Membership Queries to ensure that a host wants

to continue receiving multicast traffic. If a host does not respond, the IGMP
Snooper stops sending traffic to the host.
 The host can initiate the Leave process by sending an IGMP Leave packet to the
IGMP Snooper.
When a host sends an IGMPv2 Leave packet, the IGMP Snooper queries to find

out if any other host connected to the port is interested in receiving the multicast
traffic. If it does not receive a Join message in response, the IGMP Snooper
removes the group entry and passes on the information to the Mrouter.
The G8264 supports the following:
IGMP version 1, 2, and 3

 IGMP version 3 in stand‐alone (non‐stacking) mode only
 128 static Mrouters and 128 dynamic Mrouters
Note: Unknown multicast traffic is sent to all ports if the flood option is enabled
and no Membership Report was learned for that specific IGMP group. If the flood
option is disabled, unknown multicast traffic is discarded if no hosts or Mrouters
are learned on a switch.
To enable or disable IGMP flood, use the following command:
# vlan <vlan ID>
RS G8264(config)
RS G8264(config-vlan)
# [no] flood
Chapter 32: Internet Group Management Protocol
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