Multicast Protocols - 3Com 4210 9-Port Configuration Manual

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Ethernet multicast MAC address

When a unicast IP packet is transported in an Ethernet network, the destination MAC address is the

MAC address of the receiver. When a multicast packet is transported in an Ethernet network, a

multicast MAC address is used as the destination address because the destination is a group with an

uncertain number of members.

As stipulated by IANA, the high-order 24 bits of a multicast MAC address are 0x01005e, while the

low-order 23 bits of a MAC address are the low-order 23 bits of the multicast IP address.

describes the mapping relationship:

Figure 1-4 Multicast address mapping

32-bit IP address

48-bit MAC address

25-bit MAC address prefix

The high-order four bits of the IP multicast address are 1110, representing the multicast ID. Only 23 bits

of the remaining 28 bits are mapped to a MAC address. Thus, five bits of the multicast IP address are

lost. As a result, 32 IP multicast addresses are mapped to the same MAC address.

Multicast Protocols

Generally, we refer to IP multicast working at the network layer as Layer 3 multicast and the

corresponding multicast protocols as Layer 3 multicast protocols, which include IGMP, PIM, and

MSDP; we refer to IP multicast working at the data link layer as Layer 2 multicast and the

corresponding multicast protocols as Layer 2 multicast protocols, which include IGMP Snooping.

This section provides only general descriptions about applications and functions of the Layer 2 and

Layer 3 multicast protocols in a network. For details about these protocols, refer to the related

chapters of this manual.

Layer 3 multicast protocols

Layer 3 multicast protocols include multicast group management protocols and multicast routing

describes where these multicast protocols are in a network.

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