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 duplicate this multicast data
and deliver each port one copy. The general format of the multicast address table is described
VLAN ID
Multicast IP
Port
Figure 9-3 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
four submenus:
IGMP Snooping
,
Multicast IP,
Multicast Filter
,
Packet Statistics
.
9.1
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.
IGMP Messages
The switch, running IGMP Snooping, processes the IGMP messages of different types as
follows.
1.
IGMP Query Message
IGMP query message, sent by the router, falls into two types, IGMP general query message and
IGMP group-specific-query message. The router regularly sends IGMP general message to
query if the multicast groups contain any member. When receiving IGMP leave message, the
receiving port of the router will send IGMP group-specific-query message to the multicast
group and the switch will forward IGMP group-specific-query message to check if other
members in the multicast group of the port need this multicast.
When receiving IGMP general query message, the switch will forward them to all other ports in
the VLAN owning the receiving port. The receiving port will be processed: if the receiving port
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