Address Learning

Address Learning
The layer 2 switch is responsible for
address learning
. When a switch is powered on, the MAC
filtering table is empty. When a device transmits and a frame is received on an interface, the
switch takes the source address and places it in the MAC filter table. It remembers what interface
the device is located on. The switch has no choice but to flood the network with this frame
because it has no idea where the destination device is located.
If a device answers and sends a frame back, then the switch takes the source address from
that frame, places the MAC address in the database, and associates this address with the interface
on which the frame was received. Because the switch now has two MAC addresses in the
filtering table, the devices can now make a point-to-point connection and the frames are forwarded
only between the two devices. This is what makes layer 2 switches better than hubs. In
a hub network, all frames are forwarded out all ports every time.
Figure 15.1 shows the procedures for building a MAC database.