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Internet Group Management Protocol
(IGMP)
Internet group management protocol (IGMP) is a Layer 3 multicast protocol that hosts use to join or leave
a multicast group.
Multicast is premised on identifying many hosts by a single destination IP address; hosts represented by
the same IP address are a multicast group. Multicast routing protocols (such as protocol-independent
multicast [PIM]) use the information in IGMP messages to discover which groups are active and to
populate the multicast routing table.
IGMP Implementation Information
• The Dell Networking OS supports IGMP versions 1, 2, and 3 based on RFCs 1112, 2236, and 3376,
respectively.
• The system does not support IGMP version 3 and versions 1 or 2 on the same subnet.
• Dell Networking switches cannot serve as an IGMP host or an IGMP version 1 IGMP Querier.
• The system automatically enables IGMP on interfaces on which you enable a multicast routing
protocol.
IGMP Protocol Overview
IGMP has three versions. Version 3 obsoletes and is backwards-compatible with version 2; version 2
obsoletes version 1.
IGMP Version 2
IGMP version 2 improves on version 1 by specifying IGMP Leave messages, which allows hosts to notify
routers that they no longer care about traffic for a particular group.
Leave messages reduce the amount of time that the router takes to stop forwarding traffic for a group to
a subnet (leave latency) after the last host leaves the group. In version 1 hosts quietly leave groups, and
the router waits for a query response timer several times the value of the query interval to expire before it
stops forwarding traffic.
To receive multicast traffic from a particular source, a host must join the multicast group to which the
source is sending traffic. A host that is a member of a group is called a receiver. A host may join many
groups, and may join or leave any group at any time. A host joins and leaves a multicast group by sending
an IGMP message to its IGMP Querier. The querier is the router that surveys a subnet for multicast
receivers and processes survey responses to populate the multicast routing table.
IGMP messages are encapsulated in IP packets, as shown in the following illustration.
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Internet Group Management Protocol (IGMP)