Overview

Group membership management is fundamental to the multicasting process. An arbitrary group of receivers can express interest in receiving a particular multicast stream, regardless of the physical or geographical boundaries of its members.

The purpose of IP multicast group management is to optimize a switched network‘s performance so multicast packets will only be forwarded to those ports containing multicast group hosts or multicast switch devices instead of flooding to all ports in the subnet (VLAN).

IGMP uses three key components to control multicast membership:

• Source — A server that sends an IP multicast data stream with a particular multicast destination IP and MAC address. A server may not have direct IGMP involvement, as it often does not receive a multicast stream, but only sends a multicast stream.
• Querier — A device that periodically sends out queries in search of multicast hosts on a directly connected network. If multiple queriers are present on the LAN, the querier with the lowest IP address assumes the role.
• Host — A client end station that sends one of two IGMP messages to a querier:
  • Join message — Indicates the host wants to receive transmissions associated to a particular multicast group.
  • Leave message — Indicates the host wants to stop receiving the multicast transmissions.
    

    IGMP Querier Determining Group Membership

    

As shown in IGMP Querier Determining Group Membership, a multicast query-enabled device can periodically ask its hosts if they want to receive multicast traffic. If there is more than one device on the LAN performing IP multicasting, one of these devices is elected querier and assumes the responsibility of querying the LAN for group members.

Based on the group membership information learned from IGMP, a device can determine which (if any) multicast traffic needs to be forwarded to each of its ports. At Layer 3, multicast switch devices use this information, along with a multicast routing protocol, to support IP multicasting across the Internet.

IGMP provides the final step in IP multicast delivery. It is only concerned with forwarding multicast traffic from the local switch device to group members on a directly attached subnetwork or LAN segment.

IGMP neither alters nor routes any IP multicast packets. Since IGMP is not concerned with the delivery of IP multicast packets across subnetworks, an external IP multicast device is needed if IP multicast packets have to be routed across different subnetworks.

Note

On VLANs where IGMP snooping is enabled, any received multicast stream will be flooded to the VLAN until such time as the IGMP database is populated, then stream forwarding will revert to ports with group membership only.