H3C Technologies H3C S3100V2 Series Switches - Manual (Page 11)

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Figure 3 Multicast transmission
In Figure 3 , Host B, Host D and Host E are receivers of the information. They 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:
• 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 will not increase the load of the source and will
not remarkably add to the usage of network resources.
• 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.
Features of multicast
Multicast transmission has the following features:
• A multicast group is a multicast receiver set identified by an IP multicast address. Hosts join a
multicast group to become members of the multicast group, before they can 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.
• All hosts that have joined a multicast group become members of the multicast group. 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
manage multicast group memberships on stub subnets with attached group members. A multicast
router itself can be a multicast group member.