3
Figure 3 Multicast transmission
, the multicast source sends only one copy of the information to a multicast group. Host B,
Host D, and Host E, which are information receivers, must join the multicast group. The routers on
the network duplicate and forward the information based on the distribution of the group members.
Finally, the information is correctly delivered to Host B, Host D, and Host E.
To summarize, multicast has the following advantages:
•
Advantages
over
unicast
—Multicast data is replicated and distributed until it flows to the
farthest-possible node from the source. The increase of multicast receivers will not remarkably
increase the load of the source or the usage of network resources.
•
Advantages
over
broadcast
—Multicast data is sent only to the receivers that need it. This
saves network bandwidth use and enhances network security. In addition, multicast data is not
confined to the same subnet.
Multicast features
•
A multicast group is a multicast receiver set identified by an IP multicast address. Hosts must
join a multicast group to become members of the multicast group before they receive the
multicast data addressed to that multicast group. Typically, a multicast source does not need to
join a multicast group.
•
A multicast source is an information sender. It can send data to multiple multicast groups at the
same time. Multiple multicast sources can send data to the same multicast group at the same
time.
•
The group memberships are dynamic. Hosts can join or leave multicast groups at any time.
Multicast groups are not subject to geographic restrictions.
•
Multicast routers or Layer 3 multicast devices are routers or Layer 3 switches that support Layer
3 multicast. They provide multicast routing and manage multicast group memberships on stub
subnets with attached group members. A multicast router itself can be a multicast group
member.
For a better understanding of the multicast concept, you can compare multicast transmission to the
transmission of TV programs.
