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 traffic is replicated and distributed until it flows to the
farthest-possible node from the source. The increase of receiver hosts 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 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.
• An information sender is called a "multicast source." A multicast source 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.
• Routers or Layer 3 switches that support Layer 3 multicast are called "multicast routers" or "Layer 3
multicast devices." In addition to providing the multicast routing function, a multicast router can also
manage multicast group memberships on stub subnets with attached group members. A multicast
router itself can be a multicast group member.
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