3Com 4210 PWR Configuration Manual page 194

192 C 15: M
HAPTER ULTICAST
Multicast Protocols
n
O
VERVIEW
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. Figure 58 describes the mapping relationship:
Figure 58 Multicast address mapping
32-bit IP address
48-bit MAC address
0000 0001 0000 0000
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.
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 2 multicast protocols
Layer 2 multicast protocols include IGMP Snooping and multicast VLAN. Figure 59
shows where these protocols are in the network.
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. The Switch 4210 does support IGMP snooping.
5 bits lost
XXXX X
1110 XXXX XXXX XXXX
0101 1110 0XXX XXXX
XXXX XXXX XXXX XXXX
23 bits
mapped
XXXX XXXX XXXX XXXX