8
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
IMPORTANT:
Because of the duplicate mapping from multicast IP address to multicast MAC address, the device
might inadvertently send multicast protocol packets as multicast data in Layer 2 forwarding. To avoid
this, do not use the IP multicast addresses that are mapped to multicast MAC addresses
0100-5E00-00xx and 3333-0000-00xx (where "x" represents any hexadecimal number from 0 to F).
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.
Layer 2 multicast protocols
—IGMP snooping, MLD snooping, PIM snooping, IPv6 PIM
snooping, multicast VLAN, and IPv6 multicast VLAN.
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
…
FF1E
0000
0000
0000
0000
0000
F30E
0101
0101
a
F30E
48-bit MAC address
3333
32 bits
mapped
128-bit IPv6 address
…
…
16-bit MAC
address prefix