3Com Switch 7750 Configuration Guide

Multicast Architecture 419
n
Like having reserved the private network segment 10.0.0.0/8 for unicast, IANA has
also reserved the network segments ranging from 239.0.0.0 to 239.255.255.255
for multicast. These are administratively scoped addresses. With the
administratively scoped addresses, you can define the range of multicast domains
flexibly to isolate IP addresses between different multicast domains, so that the
same multicast address can be used in different multicast domains without
causing collisions.
Ethernet multicast MAC address
When a unicast IP packet is transported in an Ethernet network, the destination
MAC address is the MAC address of the receiver. When a multicast packet is
transported in an Ethernet network, a multicast MAC address is used as the
destination address because the destination is a group with an uncertain number
of members.
As stipulated by IANA, the high-order 24 bits of a multicast MAC address are 0 x
01005e, while the low-order 23 bits of a MAC address are the low-order 23 bits of
the multicast IP address. Figure 103 describes the mapping relationship:
Figure 103 Mapping relationship between multicast IP address and multicast MAC
address
The high-order four bits of the IP multicast address are 1110, representing the
multicast ID. Only 23 bits of the remaining 28 bits are mapped to a MAC address
Thus five bits of the multicast IP address are lost. As a result, 32 IP multicast
addresses are mapped to the same MAC address.
IP Multicast Protocols IP multicast protocols include the multicast group management protocol and the
multicast routing protocol. Figure 104 describes the positions of the protocols
related to multicast in the network.
224.0.0.16 The specified subnetwork bandwidth management (SBM)
224.0.0.17 All SBMS
224.0.0.18 Virtual router redundancy protocol (VRRP)
224.0.0.19- 224.0.0.255 Other protocols
Table 329 Reserved IP multicast addresses
Class D address range Description
XXXX X
XXXX XXXX XXXX XXXX XXXX XXXX1110 XXXX
0XXX XXXX XXXX XXXX XXXX XXXX0000 0001 0000 0000 0101 1110
32-bit IP address
48-bit MAC address
5 bits lost
25-bit MAC address prefix
Ă
Ă
23 bits
mapped