3Com Switch 7750 Configuration Guide Guide

Multicast Architecture 381
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 89 describes the mapping relationship:
Figure 89 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.
224.0.0.15 All core-based tree (CBT) routers
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 302 Reserved IP multicast addresses
Class D address range Description
XXXXX
25bit MAC address prefix
XXXXXXXX XXXXXXXX XXXXXXXX1110XXXX
XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX
…… ……
32-bit IP address
48-bit MAC address
23bit
mapping
Five bits are lost
XXXXX
25bit MAC address prefix
XXXXXXXX XXXXXXXX
XXXXX
25-bit MAC address prefix
XXXXXXXX XXXXXXXX XXXXXXXX1110XXXX
XXXXXXXX
XXXXXXXX1110XXXX
XXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX
…… ……
XXXXXXXX XXXXXXXX XXXXXXXX
…… ……
23-bit
mapping
XXXXX
25bit MAC address prefix
XXXXXXXX XXXXXXXX
XXXXX
25bit MAC address prefix
XXXXXXXX XXXXXXXX XXXXXXXX1110XXXX
XXXXXXXX
XXXXXXXX1110XXXX
XXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX
…… ……
32-bit IP address
48-bit MAC address
23bit
mapping
Five bits are lost
XXXXX
25bit MAC address prefix
XXXXXXXX XXXXXXXX
XXXXX
25-bit MAC address prefix
XXXXXXXX XXXXXXXX XXXXXXXX1110XXXX
XXXXXXXX
XXXXXXXX1110XXXX
XXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX
…… ……
XXXXXXXX XXXXXXXX XXXXXXXX
…… ……
23-bit
mapping