3com 3.01.01 Manuale Utente

C

HAPTER

 6: M

ULTICAST

 P

ROTOCOL

transmitted, the destination is no longer a specific receiver but a group with 
unspecific members. Therefore, the multicast MAC address should be used. 
Multicast MAC addresses correspond to multicast IP addresses. IANA (Internet 
Assigned Number Authority) stipulates that the higher 24 bits of the multicast 
MAC address is 0x01005e and the lower 23 bits of the MAC address is the lower 
23 bits of the multicast IP address.

Figure 2   Mapping Between the Multicast IP Address and the Ethernet MAC Address

Only 23 bits of the last 28 bits in the IP multicast address is mapped to the MAC 
address. Therefore the 32 IP multicast addresses are mapped to the same MAC 
address.

Multicast uses the multicast group management protocol, and the multicast 
routing protocol. The multicast group management protocol uses Internet Group 
Management Protocol (IGMP) as the IP multicast basic signaling protocol. It is used 
between hosts and routers and enables routers to determine if members of the 
multicast group are on the network segment. The multicast routing protocol is 
used between multicast routers and creates and maintains multicast routes, and 
allows high-efficient multicast packet forwarding. At present, multicast routing 
protocols mainly include PIM-SM, PIM-DM.

Tasks for configuring IP Multicast Protocols are described in the following sections:

■

■

Internet Group Management Protocol (IGMP) is the only protocol that hosts can 
use. It defines the membership establishment and maintenance mechanism 
between hosts and routers, and is the basis of the entire IP multicast. Hosts report 
the group membership to a router through IGMP and inform the router of the 
conditions of other members in the group through the directly connected host. 

If a user on the network joins a multicast group through IGMP declaration, the 
multicast router on the network will transmit the information sent to the multicast 
group through the multicast routing protocol. Finally, the network will be added to 
the multicast tree as a branch. When the host, as a member of a multicast group, 
begins receiving the information, the router queries the group periodically to 
check whether members in the group are involved. As long as one host is involved, 
the router receives data. When all users on the network quit the multicast group, 
the related branches are removed from the multicast tree.

A multicast group address has a virtual address. Unicast allows packets to be 
routed from the data source to the specified destination address. This is not 

48-bit MAC
address

32-bit IP
address

5 bits
not 
mapped

Lower 23 bits directly mapped