Whenever
an application needs to send data to more than one station, but only
to stations interested in receiving the
traffic, the application typically uses a multicast destination address.
Whereas a broadcast would target all users in a domain, multicast
addresses target a select group of all stations in the network. These
are both illustrated in Figures
and . On the
other hand, if the source transmits unicast frames, it would have to
send multiple copies of the frame, one addressed to each intended
receiver. As you can see in Figures
and , this is
a very inefficient use of network resources and does not scale well as
the number of receivers increases.
By using multicast addresses, the source transmits
only one copy of the frame into the network, and routers distribute the
multicast message to the other segments where interested receivers reside. As discussed previously, multicast addresses appear at Layers 2
and 3. A network administrator assigns the multicast Layer 3 address for
an application. The Layer 2 multicast address is then calculated from
the Layer 3 multicast address. When you configure a multicast
application, the NIC adds the multicast address to its list of valid
Media Access Control (MAC) addresses. Usually this list consists of the
built-in MAC address plus any user-configured addresses. Whenever the
station receives a frame with a matching multicast destination address,
the receiver sends the frame to the CPU.
The router examines multicast addresses at both Layers
2 and 3, whereas a switch examines only the Layer 2 address. However,
some high-end switches with Supervisor III cards and NFFCs can examine
the Layer 3 addresses as well. Otherwise, the switch simply examines the
MAC address in the frame.
|