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Multicast Address Table
The switch is forwarding multicast packets based on the multicast address table. As the
transmission of multicast packets cannot span the VLAN, the first part of the multicast address
table is VLAN ID, based on which the received multicast packets are forwarded in the VLAN
owning the receiving port. The multicast address table is not mapped to an egress port but a
group port list. When forwarding a multicast packet, the switch looks up the multicast address
table based on the destination multicast address of the multicast packet. If the corresponding
entry cannot be found in the table, the switch will broadcast the packet in the VLAN owning the
receiving port. If the corresponding entry can be found in the table, it indicates that the
destination address should be a group port list, so the switch will deliver this multicast data to
each port. The general format of the multicast address table is described as Figure 8-3 below.
VLAN ID
Multicast IP
Port
Figure 8-3 Multicast Address Table
IGMP Snooping
In the network, the hosts apply to the near router for joining (leaving) a multicast group by
sending IGMP (Internet Group Management Protocol) messages. When the up-stream device
forwards down the multicast data, the switch is responsible for sending them to the hosts.
IGMP Snooping is a multicast control mechanism, which can be used on the switch for dynamic
registration of the multicast group. The switch, running IGMP Snooping, manages and controls
the multicast group via listening to and processing the IGMP messages transmitted between the
hosts and the multicast router, thereby effectively prevents multicast groups being broadcasted
in the network.
The Multicast module is mainly for multicast management configuration of the switch, including
the following submenus: IGMP Snooping and Multicast Table.
8.1 IGMP Snooping
IGMP Snooping Process
The switch, running IGMP Snooping, listens to the IGMP messages transmitted between the
host and the router, and tracks the IGMP messages and the registered port. When receiving
IGMP report message, the switch adds the port to the multicast address table; when the switch
listens to IGMP leave message from the host, the router sends the Group-Specific Query
message of the port to check if other hosts need this multicast, if yes, the router will receive
IGMP report message; if no, the router will receive no response from the hosts and the switch
will remove the port from the multicast address table. The router regularly sends IGMP query
messages. After receiving the IGMP query messages, the switch will remove the port from the
multicast address table if the switch receives no IGMP report message from the host within a
period of time.
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