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Multicast Overview

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A good example of a broadcast message is an Address Resolution Proto-

col (ARP) request. When a host has a packet destined for a logical address
that is not located on the same network, the host must ARP for the default
gateway's MAC address so it can create the layer 2 frame and in turn send
the datagram to the router. The MAC address is obtained via an ARP
request. The ARP request is a broadcast message sent to all devices in the
broadcast domain. The router will be the device that responds to the broad-
cast message, whereas other stations will evaluate the frame but not respond.

This brings up another good point. Broadcasts can cause problems on net-

works. Because the broadcast frame is addressed to include every host, every
host must process the frame. CPU interruption occurs so that the frame may
be processed. This interruption affects other applications that are running on
the host. When unicast frames are seen by a router, a quick check is made to
identify whether the frame is intended for the host or not. If it isn't, the frame
is discarded.

Multicast

Multicast is a different beast entirely. At first glance, it appears to be a hybrid
of unicast and broadcast communication, but that isn't quite accurate. Mul-
ticast does allow point-to-multipoint communication, which is similar to
broadcasts, but it happens in a different manner. The crux of

multicast

that it allows multiple recipients to receive messages without flooding the
messages to all hosts on a broadcast domain.

Multicast works by sending messages or data to IP

multicast group

addresses. Routers then forward copies of the packet out every interface that
has hosts

subscribed

to that group address. This is where multicast differs

from broadcast messages. With unicast communication, copies of packets
are sent only to subscribed hosts.

The difference between multicast and unicast is comparable to the differ-

ence between mailing lists and SPAM. You subscribe to a mailing list when
you want to receive mail from a specific group regarding specific informa-
tion--for example, a Cisco User Group mailing list. You expect to get only
messages from other members of the group regarding topics related to the
user group. In contrast, SPAM is unsolicited mail that arrives in your inbox.
You aren't expecting it from the sender, nor are you likely to be interested in
the content.

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