H3C Technologies H3C S3100V2 Series Switches - Manual (Page 23)

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• If no entry in the forwarding table exists for the reported group, the switch creates an entry, adds the
port as a dynamic member port to the outgoing port list, and starts a member port aging timer for
that port.
• If an entry in the forwarding table exists for the reported group, but the port is not included in the
outgoing port list for that group, the switch adds the port as a dynamic member port to the outgoing
port list, and starts an aging timer for that port.
• If an entry in the forwarding table exists for the reported group and the port is included in the
outgoing port list, which means that this port is already a dynamic member port, the switch resets
the aging timer for that port.
NOTE:
A switch does not forward an IGMP report through a non-router port. This is because if the switch
forwards a report message through a member port, all the attached hosts listening to the reported
multicast address will suppress their own reports upon receiving this report according to the IGMP report
suppression mechanism on them, and this will prevent the switch from knowing whether the reported
multicast group still has active members attached to that port.
When receiving a leave message
When an IGMPv1 host leaves a multicast group, the host does not send an IGMP leave message, so the
switch cannot determine immediately that the host has left the multicast group. However, as the host stops
sending IGMP reports as soon as it leaves a multicast group, the switch deletes the forwarding entry for
the dynamic member port corresponding 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 IGMP leave message to
the multicast router.
When the switch receives an IGMP leave message on a dynamic member port, the switch first checks
whether an entry in the forwarding table exists for the group address in the message, and, if one exists,
whether the outgoing port list contains the port.
• If the entry in the forwarding table does not exist or if the outgoing port list does not contain the port,
the switch discards the IGMP leave message instead of forwarding it to any port.
• If the entry in the forwarding table exists and the outgoing port list contains the port, the switch
forwards the leave message to all router ports in the native VLAN. Because the switch cannot
determine whether any other hosts attached to the port are still monitoring that group address, the
switch does not immediately remove the port from the outgoing port list of the entry in the
forwarding table for that group. Instead, it resets the aging timer for the port.
After receiving the IGMP leave message from a host, the IGMP querier resolves the multicast group
address in the message and sends an IGMP group-specific query to that multicast group through the port
that received the leave message. After receiving the IGMP group-specific query, the switch forwards the
query through all its router ports in the VLAN and all member ports for that multicast group, and performs
the following to the port on which it received the IGMP leave message:
• If any IGMP report in response to the group-specific query is received on the port—suppose it is a
dynamic member port—before its aging timer expires, this means that a host attached to the port
is receiving or expecting to receive multicast data for that multicast group. The switch resets the
aging timer of the port.
• If no IGMP report in response to the group-specific query is received on the port before its aging
timer expires, this means that no hosts attached to the port are still monitoring that group address.