H3C S9500 Series Operation Manual page 365

Routing switches

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Operation Manual – Multicast Protocol

H3C S9500 Series Routing Switches

II. Ethernet Multicast MAC Addresses

When a unicast IP packet is transmitted on the Ethernet, the destination MAC address

is the MAC address of the receiver. However, for a multicast packet, the destination is

no longer a specific receiver but a group with unspecific members. Therefore, the

multicast MAC address should be used.

As Internet Assigned Number Authority (IANA) provisions, the high 24 bits of a

multicast MAC address are 0x01005e and the low 23 bits of a MAC address are the low

23 bits of a multicast IP address. The high twenty-fifth bit is 0, a fixed value.

Figure 1-4 Mapping between a multicast IP address and an Ethernet MAC address

The first four bits of the multicast address are 1110, representing the multicast identifier.

Among the rest 28 bits, only 23 bits are mapped to the MAC address, and the other five

bits are lost. This may results in that 32 IP addresses are mapped to the same MAC

1.2.2 IP Multicast Protocols

IP multicast protocols mainly involves multicast group management protocols and

multicast routing protocols. Their application positions are shown in Figure 1-5

Application positions of multicast-related protocols.

Class D address range

23 bits mapped

Chapter 1 IP Multicast Overview

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