Igmp Snooping - TP-Link T1700X-16TS User Manual

The high-order 16 bits of the IP multicast address are 0x3333, identifying the IPv6 multicast group.
The low-order 32 bits of the IPv6 multicast IP address are mapped to the multicast MAC address.
Multicast Address Table
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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 8-4 below.

IGMP Snooping

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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.
MLD Snooping
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Multicast Listener Discovery (MLD)snooping is applied for efficient distribution of IPv6 multicast
data to clients and routers in a Layer 2 network. With MLD snooping, IPv6 multicast data is
selectively forwarded to a list of ports that want to receive the data, instead of being flooded to all
ports in a VLAN. The list is constructed and maintained by snooping IPv6 multicast control packets.
MLD snooping performs a similar function in IPv6 as IGMP snooping in IPv4.
The Multicast module is mainly for multicast management configuration of the switch, including
three submenus: IGMP Snooping, MLD Snooping and Multicast Table.
8.1 IGMP Snooping
IGMP Snooping Process
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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.
VLAN ID Multicast IP
Figure 8-4 Multicast Address Table
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Port