H3C Technologies H3C S12500-X Series Switches - Manual (Page 27)

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permit the port that received the report to join the multicast group, the switch creates an IGMP snooping
forwarding entry for the multicast group and adds the port to the forwarding entry. Otherwise, the switch
drops this report, in which case the multicast data for the multicast group is not sent to this port, and the
user cannot retrieve the program.
A switch does not forward an IGMP report through a non-router port. If the switch forwards a report
message through a member port, the IGMP report suppression mechanism causes all attached hosts that
monitor the reported multicast address to suppress their own reports. In this case, the switch cannot
determine whether the reported multicast group still has active members attached to that port. For more
information about the IGMP report suppression mechanism, see " Configuring IGMP ."
Leave message
An IGMPv1 host silently leaves a multicast group, and the switch is not notified of the leave. However,
because the host stops sending IGMP reports as soon as it leaves the multicast group, the switch removes
the port that connects to the host from the forwarding entry for the multicast group when the aging timer
for the port expires.
An IGMPv2 or IGMPv3 host sends an IGMP leave message to the multicast router when it leaves a
multicast group.
When the switch receives an IGMP leave message on a dynamic member port, the switch first examines
whether a forwarding entry matches the group address in the message, and, if a match is found, whether
the forwarding entry for the group contains the dynamic member port.
• If no forwarding entry matches the group address, or if the forwarding entry does not contain the
port, the switch directly discards the IGMP leave message.
• If a forwarding entry matches the group address and the forwarding entry contains the port, the
switch forwards the leave message to all router ports in the VLAN. Because the switch does not
know whether any other hosts attached to the port are still listening to that group address, the switch
does not immediately remove the port from the forwarding entry for that group. Instead, it adjusts
the aging timer for the port.
After receiving the IGMP leave message, the IGMP querier resolves the multicast group address in the
message and sends an IGMP group-specific query to the multicast group through the port that received
the leave message. After receiving the IGMP group-specific query, the switch forwards it through all its
router ports in the VLAN and all member ports of the multicast group. The switch also performs the
following judgment for the port that received the IGMP leave message:
• If the port (assuming that it is a dynamic member port) receives an IGMP report in response to the
group-specific query before its aging timer expires, it indicates that some host attached to the port
is receiving or expecting to receive multicast data for the multicast group. The switch restarts the
aging timer for the port.
• If the port receives no IGMP report in response to the group-specific query before its aging timer
expires, it indicates that no hosts attached to the port are still listening to that group address. The
switch removes the port from the forwarding entry for the multicast group after the aging timer for
the port expires.
Protocols and standards
RFC 4541, Considerations for Internet Group Management Protocol (IGMP) and Multicast Listener
Discovery (MLD) Snooping Switches