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. Figure 1-4 describes the mapping relationship:

Figure 1-4Multicast address mapping

32-bit IP address

48-bit MAC address

0000 0001 0000 0000

5 bits lost

XXXX X

1110 XXXX XXXX XXXX

0101 1110

 

0XXX XXXX

 

 

 

XXXX XXXX

23bits mapped

XXXX XXXX

XXXX XXXX

XXXX XXXX

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

zGenerally, 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.

zThis 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 protocols. Figure 1-5 describes where these multicast protocols are in a network.

1-9

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3Com WX3000 operation manual Multicast Protocols, Layer 3 multicast protocols