Bridges & Switches

Network devices have one primary purpose: to pass network traffic from one segment to another. (There are exceptions, of course, such as network analyzers, which inspect traffic as it goes by.) With devices that independently make forwarding decisions, traffic can travel from its source to the destination. The higher up the Open System Interconnection (OSI) model a device operates, the deeper it looks into a packet to make a forwarding decision.

Bridges and switches are networking devices that operate at OSI Layer 2. Bridges became popular in the 1980s and enabled packet forwarding between homogenous networks. More recently, bridges and switches forward frames among different types of networks.

Switching technology has emerged as the evolutionary replacement for bridging. Switches provide all the features of traditional bridging and more. Compared to bridges, switches provide superior throughput performance, higher port density, and lower per-port cost.

The different types of bridging include the following:

• Transparent bridging primarily occurs in Ethernet networks.
• Source-route bridging occurs in Token Ring networks.
• Translational bridging occurs between different media. For example, a translational bridge might connect a Token Ring network to an Ethernet network.

Bridging and switching occur at the data link layer, which means bridges control data flow, provide transmission error handling, and enable access to physical media. Basic bridging is not complicated: A bridge or switch analyzes an incoming frame, determines where to forward the frame based on the packet’s content, and forwards the frame toward its destination. With transparent bridging, forwarding decisions happen one hop at a time. With source-route bridging, the frame contains a predetermined path to the destination.

Bridges and switches divide networks into smaller, self-contained units. Because only a portion of traffic is forwarded, bridging reduces the overall traffic devices see on each connected network. The bridge acts as a kind of firewall in that it prevents frame-level errors from propagating from one segment to another. Bridges also accommodate communication among more devices than are supported on a single segment or ring. Bridges and switches essentially extend the effective length of a LAN, permitting more workstations to communicate with each other within a single broadcast domain.

The primary difference between switches and bridges is that bridges segment a LAN into a few smaller segments. Switches, through their increased port density and speed, permit segmentation on a much larger scale. Modern-day switches have hundreds of ports per chassis. Additionally, modern-day switches interconnect LAN segments operating at different speeds.