Operation Manual – MulticastH3C S7500E Series Ethernet Switches Chapter 2 IGMP Snooping Configuration2-4Note:The port aging mechanism of IGMP Snooping works only for dynamic ports; a staticport will never age out.2.1.3 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(224.0.0.1) on the local subnet to find out whether active multicast group membersexist on the subnet.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 reported multicast group, and performs thefollowing:z If no forwarding table entry exists for the reported group, the switch creates anentry, adds the port as member port to the outgoing port list, and starts a memberport aging timer for that port.z If a forwarding table entry exists for the reported group, but the port is not includedin the outgoing port list for that group, the switch adds the port as a member port tothe outgoing port list, and starts a member port aging timer for that port.
