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15 Layer 2 Switching Overview
15.1 Overview
15.1.1 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 of a frame, the frame is forwarded to the
port in the entry (however, if the port in the entry is the port on which the frame was received,
the frame is not forwarded). 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.
15.1.2 VLAN
VLAN functionality divides a switch into virtual groups. A switch can be internally grouped
into multiple VLANs to partition broadcast domains. This allows enhanced broadcast frame
control and security.
The figure below provides a VLAN overview. Because the broadcast domain is divided
between VLAN A and VLAN B, no frames will arrive.
Figure 15-1 VLAN overview
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