Layer 2 switches segment a network into multiple collision domains and interconnect devices within a workgroup, such as a group of PCs.
The heart of a Layer 2 switch is its Media Access Control (MAC) address table, also known as its content-addressable memory (CAM). This table contains a list of the MAC addresses that are reachable through each switch port. (Recall that the physical MAC address uniquely identifies a device on a network. When a network interface card is manufactured, the card is assigned an addresscalled a burned-in address [BIA]which doesn't change when the network card is installed in a device and is moved from one network to another. Typically, this BIA is copied to interface memory and is used as the MAC address of the interface.) The MAC address table can be statically configured, or the switch can learn the MAC addresses dynamically. When a switch is first powered up, its MAC address table is empty, as shown in the example network of Figure 2-1.
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