H3C Technologies H3C S12500 Series Switches - Manual (Page 24)

8
Bit Description
T
• When set to 0, this address is an IPv6 multicast address
permanently-assigned by IANA.
• When set to 1, this address is a transient, or dynamically
assigned IPv6 multicast address.
{
Scope—Contains four bits, which represent the scope of the IPv6 internetwork for which the
multicast traffic is intended.
Table 5 Scope field values
Value Meaning
0, 3, F Reserved
1 Interface-local scope
2 Link-local scope
4 Admin-local scope
5 Site-local scope
6, 7, 9 through D Unassigned
8 Organization-local scope
E Global scope
{
Group ID—Contains 112 bits. It uniquely identifies an IPv6 multicast group in the scope that the
Scope field defines.
Ethernet multicast MAC addresses
A multicast MAC address identifies a group of receivers at the data link layer.
• 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 a multicast IPv4 address.
Figure 6 IPv4-to-MAC address mapping
The most significant four bits of a multicast IPv4 address are 1110, so this address is a multicast
address. Only 23 bits of the remaining 28 bits are mapped to a MAC address, so 5 bits of the
multicast IPv4 address are lost. As a result, 32 multicast IPv4 addresses map to the same IPv4
multicast MAC address. Therefore, in Layer 2 multicast forwarding, a device might receive some
multicast data destined for other IPv4 multicast groups. The upper layer must filter such redundant
data.