| Not all frames contain unicast
destination addresses. Some have broadcast or multicast destination
addresses. Workstations and network devices treat broadcast and
multicast frames differently from unicast frames. Stations view
broadcast frames as public service announcements. When a station
receives a broadcast, the source is saying, "Pay attention! I
might have an important message for you!"
A broadcast frame has a destination
MAC address of FF-FF-FF-FF-FF-FF (all binary 1s). Like unicast
frames, all stations receive a frame with a broadcast destination
address. When the interface compares its own MAC address against the
destination address, they don't match. Normally, a station discards
the frame because the destination address does not match its own
hardware address. But broadcast frames are treated differently. Even
though the destination and built-in address don't match, the
interface module is designed so that it still passes the broadcast
frame to the processor. This is intentional because the broadcast
frame might have an important request or information. Unfortunately,
probably only one or at most a few stations really need to receive
the broadcast message.
For example, an IP ARP request
creates a broadcast frame, even though it intends for only one
station to respond. The source sends the request as a broadcast
because it does not know the destination MAC address and is
attempting to find it. When a source only knows the destination IP address it creates an ARP request. However, that is not enough information to address a station on the LAN.
The frame must
also contain the destination MAC address.
Routing protocols sometimes use
broadcast MAC addresses when they announce their routing tables. For
example, by default, routers send IP Routing Information Protocol
(RIP) updates every 30 seconds. The router transmits the update in a
broadcast frame. The router does not necessarily know all the
routers on the network. By sending a broadcast message, the router
is sure that all routers attached to the network will receive the
message. There is a downside to this, however. All devices on the
LAN receive and process the broadcast frame, even though only a few
devices really needed the updates. This consumes CPU cycles in every
device. If the number of broadcasts in the network becomes
excessive, workstations cannot do the things they need to do, such
as run word processors or flight simulators.
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