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| 1.2 |  | Key Characteristics of Various Switching Technologies |
| 1.2.2 |  | Layer 2 switching |

Bridges and switches analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames toward the destination. Upper-layer protocol transparency is a primary advantage of both bridging and switching. Because both device types operate at the data link layer, they are not required to examine upper-layer information. Bridges are also capable of filtering frames based on any Layer 2 fields.

Although bridges and switches share most relevant attributes, several distinctions differentiate these technologies. Switches are significantly faster because they switch in hardware. Switches also can support higher port densities than bridges. Some switches support cut-through switching, which reduces latency and delays in the network, while bridges support only store-and-forward traffic switching. The main differences are that bridges perform switching through software, as opposed to hardware, and switches have a higher port density.

Layer 2 switching is basically hardware-based bridging . In a switch, frame forwarding is handled by specialized hardware called Application-Specific Integrated Circuits (ASICs). The ASIC technology engineered for switches allows for scalability up to gigabit speeds, with low latency at costs significantly lower than Ethernet bridges.

Layer 2 switches provide network managers with the ability to increase bandwidth without adding complexity to the network. Layer 2 data frames consist of both control information, such as MAC addresses, and end-user content. At Layer 2, no modification of the frame control information is required when moving between similar Layer 1 interfaces, such as Ethernet and Fast Ethernet. However, changes to control information may occur when bridging between unlike media types such Fiber Distributed Data Interface (FDDI) or ATM and Ethernet.

Workgroup connectivity and network segmentation are the two primary uses for Layer 2 switches. The high performance of a Layer 2 switch allows for network designs that significantly decrease the number of hosts per physical segment creating a flatter design with more segments in the campus network.

Despite the advantages of Layer 2 switching, it still has all the same characteristics and limitations of legacy bridging. Broadcast domains are unaffected by the incorporation of Layer 2 switches. The same scaling and performance issues exist in this respect as in the large bridged networks of the past. The broadcast and multicast radius increases with the number of hosts, and broadcasts are still propagated across the network.

It is evident that Layer 3 functionality is still needed within the network. The next section introduces Layer 3 switching.