H3C Technologies H3C S3100 Series Switches - Manual (Page 282)

1-9
Value Meaning
E Global scope
Group ID: 112 bits, IPv6 multicast group identifier that uniquely identifies an IPv6 multicast group in the
scope defined by the Scope field.
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.
1) IPv4 multicast MAC addresses
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 1-6
describes the mapping relationship:
Figure 1-6 Multicast address mapping
XXXX X
XXXX XXXX XXXX XXXX XXXX XXXX 1110 XXXX
0XXX XXXX XXXX XXXX XXXX XXXX 0000 0001 0000 0000 0101 1110
32-bit IP address
48-bit MAC address
5 bits lost
25-bit MAC address prefix
… … 23 bits
mapped
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.
2) IPv6 multicast MAC addresses
The high-order 16 bits of an IPv6 multicast MAC address are 0x3333, and the low-order 32 bits are the
low-order 32 bits of a multicast IPv6 address. Figure 1-7 shows an example of mapping an IPv6
multicast address, FF1E::F30E:101, to a MAC address.
Figure 1-7 An example of IPv6-to-MAC address mapping