3member ports. The switch records all member ports on the local device in the IGMP Snoopingforwarding table.Port aging timers in IGMP Snooping and related messages and actionsTable 6-1 Port aging timers in IGMP Snooping and related messages and actionsTimer Description Message beforeexpiry Action after expiryRouter portaging timerFor each router port, theswitch sets a timer initializedto the aging time of the routeportIGMP general query orPIM helloThe switch removesthis port from its routerport listMember portaging timerWhen a port joins a multicastgroup, the switch sets a timerfor the port, which isinitialized to the member portaging timeIGMP membershipreportThe switch removesthis port from themulticast groupforwarding tableWork Mechanism of IGMP SnoopingA switch running IGMP Snooping performs different actions when it receives different IGMP messages,as follows:When receiving a general queryThe IGMP querier periodically sends IGMP general queries to all hosts and routers on the local subnetto find out whether active multicast group members exist on the subnet.Upon receiving an IGMP general query, the switch forwards it through all ports in the VLAN except thereceiving 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 resets the aging timer ofthis router port.z If the receiving port is not a router port existing in its router port list, the switch adds it into its routerport list and sets an aging timer for this router port.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 an IGMP report.z When intended to join a multicast group, a host sends an IGMP report to the multicast router toannounce that it is interested in the multicast information addressed to that group.Upon receiving an IGMP report, the switch forwards it through all the router ports in the VLAN, resolvesthe address of the multicast group the host is interested in, and performs the following to the receivingport:z If the port is already in the forwarding table, the switch resets the member port aging timer of theport.z If the port is not in the forwarding table, the switch installs an entry for this port in the forwardingtable and starts the member port aging timer of this port.
