Operation Manual – MulticastH3C S7500 Series Ethernet Switches Chapter 6 IGMP Configuration6-2membership query messages. In this case, the querier selection mechanism isrequired to specify a router as the querier.In IGMPv1, the multicast routing protocol selects the querier. In IGMPv2, it is definedthat the multicast router with the lowest IP address is selected as the querier whenthere are multiple multicast routers in a network segment.II. Leave group mechanismIn IGMPv1, hosts leave the multicast group quietly without informing any multicastrouter. Only when a query message times out can the multicast router know that a hosthas left the group. In IGMPv2, when a host replying to the last membership querymessage decides to leave a multicast group, it will send a leave group message to themulticast router.III. Group-specific queryIn IGMPv1, a multicast query message of the multicast router aims at all the multicastgroups in the network segment. This query is called general query.IGMPv2 adds group-specific query, where the IP address of a multicast group is takenas the destination IP address and the group address field of the query message, toprevent the member hosts of other groups from responding to this message.IV. Maximum response timeThe Maximum Response Time field is added in IGMPv2. It is used to dynamicallyadjust the maximum time for a host to respond to the membership query message.6.1.3 Working Procedure of IGMPThe working procedure of IGMP is as follows:z The receiver host reports the membership to its shared network.z A querier (IGMPv2) is selected from all the IGMP-enabled routers in the samenetwork segment.z The querier periodically sends query messages to the shared network segment.z The receiver host responds to the received query message to report the groupmembership.z The querier refreshes the presence information of the group members accordingto the received responses.IGMP must be enabled on all the multicast receiver hosts. An IP multicast host can joinin or exit a multicast group anywhere and anytime, without being restricted by the totalnumber of group members.The multicast router need not and cannot save the membership information of all thehosts. It just checks the network segment connected with each interface by IGMP tosee whether there are receivers of a multicast group, namely, group members. Whileeach host saves only the information about the multicast groups it joins.
