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.
Table 1: Comparing TV program transmission and multicast transmission
TV program transmission | Multicast transmission |
|---|---|
A TV station transmits a TV program through a channel. | A multicast source sends multicast data to a multicast group. |
A user tunes the TV set to the channel. | A receiver joins the multicast group. |
The user starts to watch the TV program transmitted by the TV station on the channel. | The receiver starts to receive the multicast data sent by the source to the multicast group. |
The user turns off the TV set or tunes to another channel. | The receiver leaves the multicast group or joins another group. |