Multicast Protocols - H3C s3100-52P Operation Manual

Operation Manual – Multicast
H3C S3100-52P Ethernet Switch
II. 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 0x01005e,
while the low-order 23 bits of a MAC address are the low-order 23 bits of the multicast
IP address.
48-bit MAC address
0000 0001
Figure 1-4 Multicast address mapping
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.

1.3.1 Multicast Protocols

Note:
Generally, we refer to IP multicast working at the network layer as Layer 3 multicast
and the corresponding multicast protocols as Layer 3 multicast protocols, which
include IGMP, PIM, and MSDP; we refer to IP multicast working at the data link layer
as Layer 2 multicast and the corresponding multicast protocols as Layer 2 multicast
protocols, which include IGMP Snooping.
This section provides only general descriptions about applications and functions of
the Layer 2 and Layer 3 multicast protocols in a network. For details about these
protocols, refer to the related chapters of this manual.
Figure 1-4 describes the mapping relationship:
1110 XXXX
32-bit IPv4 address
0000 0000 0101 1110
25-bit MAC address prefix
5 bits lost
XXXX X
XXXX XXXX XXXX XXXX
...
mapped
0XXX XXXX XXXX XXXX
1-10
Chapter 1 Multicast Overview
XXXX XXXX
23 bits
...
XXXX XXXX