3Com Switch 8800 Family Configuration Guide

Implementation of IP Multicast 413
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 99 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 address.
IP Multicast Protocols IP multicast protocols mainly involves multicast group management protocols and
multicast routing protocols. Their application positions are shown in
Figure 100
“Application positions of multicast-related protocols”.
224.0.0.13 All PIM routers
224.0.0.14 RSVP encapsulation
224.0.0.15 All CBT routers
224.0.0.16 Specified SBM
224.0.0.17 All SBMS
224.0.0.18 VRRP
...... ......
Table 408 Reserved multicast address list
Class D address range Description
1110XXXX XXXXXXXX XXXXXXXX XXXXXX XX
XXXXXX XX XXXXXXXX XX X XXXXX XXXXXXXXXXXXXX XX XXXXXX XX
32-bit IP
address
5 bits
unmapped
23 bits mapped
48-bit MAC
address