Operation Manual – MulticastH3C S3100 Series Ethernet Switches Chapter 2 IGMP Snooping Configuration2-3Timer Description Messagebefore expiryAction afterexpiryMember portaging timerWhen a port joins amulticast group, theswitch sets a timer forthe port, which isinitialized to the memberport aging timeIGMPmembershipreportThe switchremoves this portfrom the multicastgroup forwardingtable2.1.3 Work Mechanism of IGMP SnoopingA switch running IGMP Snooping performs different actions when it receives differentIGMP messages, as follows:I. When receiving a general queryThe IGMP querier periodically sends IGMP general queries to all hosts and routers onthe local subnet to find out whether active multicast group members exist on thesubnet.Upon receiving an IGMP general query, the switch forwards it through all ports in theVLAN except the receiving port and performs the following to the receiving port:z If the receiving port is a router port existing in its router port list, the switch resetsthe aging timer of this router port.z If the receiving port is not a router port existing in its router port list, the switch addsit into its router port list and sets an aging timer for this router port.II. When receiving a membership reportA host sends an IGMP report to the multicast router in the following circumstances:z Upon receiving an IGMP query, a multicast group member host responds with anIGMP report.z When intended to join a multicast group, a host sends an IGMP report to themulticast router to announce that it is interested in the multicast informationaddressed to that group.Upon receiving an IGMP report, the switch forwards it through all the router ports in theVLAN, resolves the address of the multicast group the host is interested in, andperforms the following to the receiving port:z If the port is already in the forwarding table, the switch resets the member portaging timer of the port.z If the port is not in the forwarding table, the switch installs an entry for this port inthe forwarding table and starts the member port aging timer of this port.
