HP 6125G HP 6125G & 6125G/XG Blade Switches IP Multicast Configuration - Page 19

Table 5 Values of the Scope field Value 0, F 1 2 3 4 5 6, 7, 9 through D 8 E Meaning Reserved Interface-local scope Link-local scope Subnet-local scope Admin-local scope Site-local scope Unassigned Organization-local scope Global scope { 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 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, which indicates that this address is a multicast address. Only 23 bits of the remaining 28 bits are mapped to a MAC address, so five 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 switch might receive some multicast data destined for other IPv4 multicast groups. The upper layer must filter such redundant data. • IPv6 multicast MAC addresses The most significant 16 bits of an IPv6 multicast MAC address are 0x3333. The least significant 32 bits are the least significant 32 bits of a multicast IPv6 address. 8