From a network efficiency standpoint, it is easy to
see that it is much better for the network when the
switch knows all the addresses on every port.
However, it is not always practical to enter this
information manually. As the network grows and
changes are made, it becomes almost impossible to
keep up.
A switch always does something when it receives
traffic. The preference is to send the traffic out a
specific port (called filtering), but this works only
when the location of the intended destination is
known. When the destination address is unknown,
the switch forwards the traffic out every port,
except the one on which the traffic was received.
This process is called flooding. Think of this as a
guy calling every number in the phone book because
he lost a woman’s number from the night before. 61