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After receiving an IGMP report, the switch forwards it through all the router ports in the VLAN, resolves
the address of the reported multicast group. The switch also performs the following judgment:
•
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. The reason is that if the switch
forwards a report message through a member port, all the attached hosts that are monitoring the
reported multicast address suppress their own reports after receiving this report according to the IGMP
report suppression mechanism. This prevents the switch from determining whether the reported
multicast group still has active members attached to that port. For more information about the IGMP
report suppression mechanism, see the chapter “IGMP configuration.”
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 that corresponds 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
determines 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. The switch
also performs the following judgment on the port that received the IGMP leave message:
•
If the port (a dynamic member port supposed) receives any IGMP report in response to the group-
specific query before its aging timer expires, it indicates 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.
Summary of Contents for A5500 EI Switch Series
Page 12: ...xii Conventions 425 Index 427 ...
