3Com SuperStack 4 Configuration Manual page 156

168 C 7: M
HAPTER ULTICAST
IP Multicast Protocols
P
ROTOCOL
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 38 Mapping between the multicast IP address and the Ethernet MAC address
32 bits IP address
48 bits MAC address
XXXX XXXX
X X X X XXXX
Only 23 bits of the last 28 bits in the IP multicast address are 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 to create and maintain multicast routes, enabling
highly-efficient multicast packet forwarding. Multicast routing protocols
supported by the Switch 5500G-EI include PIM-SM and PIM-DM.
Internet Group Management Protocol (IGMP)
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 will query 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.
Multicast Routing Protocol
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
possible for multicast. The multicast application sends the packets to a group of
receivers (as with multicast addresses) who are ready to receive the data but not
only to one receiver (as with unicast address).
The multicast routing creates a loop-free data transmission path from one data
source to multiple receivers. The task of the multicast routing protocol is to create
111 0 XXXX
XX XX XXX X
5 bits not mapped
Lower 23 bits directly mapped
XXXX XXXX
XXX X XXXX
XXXX XXXX
XXXX X X X X
XXXX XXXX
XX XX X X X X