Operation Manual – MulticastH3C S7500 Series Ethernet Switches Chapter 6 IGMP Configuration6-1Chapter 6 IGMP Configuration6.1 Overview6.1.1 Introduction to IGMPInternet group management protocol (IGMP) is responsible for the management of IPmulticast members. It is used to establish and maintain membership between IP hostsand their directly connected neighboring multicast routers.However, the IGMP feature does not transmit and maintain the membershipinformation between multicast routers. This task is completed by multicast routingprotocols. All multicast host must have IGMP enabled.IGMP is divided into two function parts:z Host side: an IP multicast host can join or exit a multicast group anywhere andanytime, without being restricted by the total number of group members.z Router side: through the IGMP protocol, a multicast router checks the networksegment connected with each interface to see whether there are receivers of amulticast group, namely, group members.A multicast router need not and cannot save the membership information of all thehosts. However, a host must save the information about the multicast groups it joins in.IGMP is asymmetric between the host and the router. The host needs to use IGMPreport messages to respond to the IGMP query messages of the multicast routers. Themulticast router sends IGMP general query messages periodically and determineswhether any host of a specified group joins in its subnet based on the receivedresponse messages. When the router receives IGMP leave messages, it will sendIGMPv2 group-specific query messages to find out whether the specified group still hasany member.6.1.2 IGMP VersionIGMP has three versions until now, including: IGMPv1 defined by RFC1112, IGMPv2defined by RFC2236 and IGMPv3. IGMPv2 is the most widely used currently.Compared with IGMPv1, the advantages of IGMPv2 are:I. Multicast router election mechanism on a shared network segmentA shared network segment is a network segment with multiple multicast routers. In thiscase, all routers running IGMP on this network segment can receive the membershipreport messages from hosts. Therefore, only one router is necessary to send
