Operation Manual – MulticastH3C S3100 Series Ethernet Switches Chapter 2 IGMP Snooping Configuration2-4Note:A switch will not forward an IGMP report through a non-router port for the followingreason: Due to the IGMP report suppression mechanism, if member hosts of thatmulticast group still exist under non-router ports, the hosts will stop sending reportswhen they receive the message, and this prevents the switch from knowing if membersof that multicast group are still attached to these ports.III. When receiving a leave messageWhen an IGMPv1 host leaves a multicast group, the host does not send an IGMP leavemessage, so the switch cannot know immediately that the host has left the multicastgroup. However, as the host stops sending IGMP reports as soon as it leaves amulticast group, the switch deletes the forwarding entry for the member portcorresponding to the host from the forwarding table when its aging timer expires.When an IGMPv2 or IGMPv3 host leaves a multicast group, the host sends an IGMPleave message to the multicast router to announce that it has leaf the multicast group.Upon receiving an IGMP leave message on the last member port, a switch forwards itout all router ports in the VLAN. Because the switch does not know whether any othermember hosts of that multicast group still exists under the port to which the IGMP leavemessage arrived, the switch does not immediately delete the forwarding entrycorresponding to that port from the forwarding table; instead, it resets the aging timer ofthe member port.Upon receiving the IGMP leave message from a host, the IGMP querier resolves fromthe message the address of the multicast group that the host just left and sends anIGMP group-specific query to that multicast group through the port that received theleave message. Upon receiving the IGMP group-specific query, a switch forwards itthrough all the router ports in the VLAN and all member ports of that multicast group,and performs the following to the receiving port:z If any IGMP report in response to the group-specific query arrives to the memberport before its aging timer expires, this means that some other members of thatmulticast group still exist under that port: the switch resets the aging timer of themember port.z If no IGMP report in response to the group-specific query arrives to the memberport before its aging timer expires as a response to the IGMP group-specific query,this means that no members of that multicast group still exist under the port: theswitch deletes the forwarding entry corresponding to the port from the forwardingtable when the aging timer expires.
