H3C S5500-EI Series Operation Manual page 16

Operation Manual – Multicast
H3C S5500-EI Series Ethernet Switches
1) IPv4 multicast MAC addresses
As defined by IANA, the high-order 24 bits of an IPv4 multicast MAC address are
0x01005e, bit 25 is 0x0, and the low-order 23 bits are the low-order 23 bits of a
multicast IPv4 address. The IPv4-to-MAC mapping relation is shown in
Figure 1-5 IPv4-to-MAC address mapping
The high-order four bits of a multicast IPv4 address are 1110, indicating that this
address is a multicast address, and 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 MAC address. Therefore, in Layer 2
multicast forwarding, a device may receive some multicast data addressed for other
IPv4 multicast groups, and such redundant data needs to be filtered by the upper layer.
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.
example of mapping an IPv6 multicast address, FF1E::F30E:0101, to a MAC address.
Figure 1-6 An example of IPv6-to-MAC address mapping
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Chapter 1 Multicast Overview
Figure 1-5.
Figure 1-6 shows an