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| 7.2 |  | Examining Multicast in a Multilayer Switched Network |
| 7.2.5 |  | IGMP and CGMP operation |

Subscribing and Maintaining Groups
The Internet Group Management Protocol (IGMP) grew out of the Host Membership Protocol, and the first version, IGMPv1, was defined in RFC 1112. The most recent version, IGMPv2, was ratified in November 1997 as a standard by the Internet Engineering Task Force (IETF) and is covered in RFC 2236.

IGMP messages are used primarily by multicast hosts to signal their local multicast router when they wish to join a specific multicast group and begin receiving group traffic, as shown in Figure . Hosts may also (with IGMPv2) signal to the local multicast router that they wish to leave an IP multicast group and are no longer interested in receiving multicast group traffic.

Using information obtained through IGMP, routers maintain a list of multicast group memberships on a per-interface basis. A multicast group membership is active on an interface if at least one host on that interface has signaled its desire, using IGMP, to receive the multicast group traffic.