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Figure 9-3 Mapping relationship between multicast IPv6 address and multicast IPv6 MAC

address

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

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 9-4 below.

VLAN ID

Multicast IP

Port

Figure 9-4 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.



MLD Snooping

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

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