H3C Technologies H3C S12500-X Series Switches - Manual (Page 13)

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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—Because multicast traffic flows to the farthest-possible node from the
source before it is replicated and distributed, an increase in the number of hosts does not increase
the load of the source or remarkably add to the usage of network resources.
• Advantages over broadcast—Because multicast data is sent only to the receivers that need it,
multicast uses network bandwidth reasonably and enhances network security. In addition, data
broadcast is confined to the same subnet, but multicast is not.
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, and 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.