24.1.1 MAC address-learning

When a Layer 2 switch receives a frame, it registers the source MAC address in a MAC address table. Each entry in the MAC address table contains the MAC address and port on which the frame was received, as well as an aging timer. Each time a frame is received, the entry corresponding to the source MAC address is updated.

Layer 2 switches forward frames according to the entries in the MAC address table. When an entry matches the destination MAC address, the frame is forwarded to the port in the entry only if the port in the entry matches the port on which the frame was received. If no entries match, the frame is forwarded to all ports other than the one on which the frame was received. This kind of forwarding is called flooding.