8
Group ID
—The Group ID field contains 112 bits. It uniquely identifies an IPv6 multicast
group in the scope that the Scope field defines.
Ethernet multicast MAC addresses
•
IPv4 multicast MAC addresses:
As defined by IANA, the most significant 24 bits of an IPv4 multicast MAC address are
0x01005E. Bit 25 is 0, and the other 23 bits are the least significant 23 bits of an IPv4 multicast
address.
Figure 6 IPv4-to-MAC address mapping
The most significant four bits of an IPv4 multicast address are fixed at 1110. In an IPv4-to-MAC
address mapping, five bits of the IPv4 multicast address are lost. As a result, 32 IPv4 multicast
addresses are mapped to the same IPv4 multicast MAC address. A device might receive
unwanted multicast data at Layer 2 processing, which needs to be filtered by the upper layer.
•
IPv6 multicast MAC addresses:
As defined by IANA, the most significant 16 bits of an IPv6 multicast MAC address are 0x3333.
The least significant 32 bits are mapped from the least significant 32 bits of an IPv6 multicast
address. Therefore, the problem of duplicate IPv6-to-MAC address mapping also arises like
IPv4-to-MAC address mapping.
Figure 7 IPv6-to-MAC address mapping
Multicast protocols
Multicast protocols include the following categories:
•
Layer 3 and Layer 2 multicast protocols:
Layer 3 multicast refers to IP multicast operating at the network layer.
Layer 3 multicast protocols
—IGMP, MLD, PIM, IPv6 PIM, MSDP, MBGP, and IPv6
MBGP.
Layer 2 multicast refers to IP multicast operating at the data link layer.
XXXX X
X
XXX XXXX
XXXX XXXX
XXXX XXXX
1110
XXXX
0XXX XXXX
XXXX XXXX
XXXX XXXX
0000 0001
0000 0000
0101 1110
32-bit IPv4 address
48-bit MAC address
5 bits lost
25-bit MAC address prefix
…
23 bits
mapped
…
F F 1 E
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
0 0 0 0
F 3 0 E
0 1 0 1
0 1 0 1
a
F 3 0 E
48-bit MAC address
3 3 3 3
32 bits
mapped
128-bit IPv6 address
…
…
16-bit MAC
address prefix
