Switches were initially introduced to provide higher-performance connectivity than hubs because switches define multiple collision domains. Switches have always been able to process data at a faster rate than routers because the switching functionality is implemented in hardware—in Application-Specific Integrated Circuits (ASIC)—rather than in software, which is how routing has traditionally been implemented. However, switching was initially restricted to the examination of Layer 2 frames. With the advent of more powerful ASICs, switches can now process Layer 3 packets, and even the contents of those packets, at high speeds.
The following sections first examine the operation of traditional Layer 2 switching. Layer 3 switching—which is really routing in hardware—is then explored.
Layer 2 Switching
Layer 2 LAN 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 MAC address table, also known as its content-addressable memory. This table contains a list of the MAC addresses that are reachable through each switch port. Recall that a physical MAC address uniquely identifies a device on a network. When a switch is first powered up, its MAC address table is empty, as shown in Figure 1-16.
Figure 1-16 The MAC Address Table Is Initially Empty
MAC Address Table Port Addresses that can be reached
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