C H A P T E R 4
• Learn about different LAN protocols.
• Learn about the different methods used to deal with media contention.
• Learn about different LAN topologies.
Bridging and Switching Basics
This chapter introduces the technologies employed in devices loosely referred to as bridges and
switches. Topics summarized here include general link layer device operations, local and remote
bridging, ATM switching, and LAN switching. Chapters in Part V, “Bridging and Switching,” address
specific technologies in more detail.
What Are Bridges and Switches?
Bridges and switches are data communications devices that operate principally at Layer 2 of the OSI
reference model. As such, they are widely referred to as data link layer devices.
Bridges became commercially available in the early 1980s. At the time of their introduction, bridges
connected and enabled packet forwarding between homogeneous networks. More recently, bridging
between different networks has also been defined and standardized.
Several kinds of bridging have proven important as internetworking devices. Transparent bridging is
found primarily in Ethernet environments, while source-route bridging occurs primarily in Token Ring
environments. Translational bridging provides translation between the formats and transit principles of
different media types (usually Ethernet and Token Ring). Finally, source-route transparent bridging
combines the algorithms of transparent bridging and source-route bridging to enable communication in
mixed Ethernet/Token Ring environments.
Today, switching technology has emerged as the evolutionary heir to bridging-based internetworking
solutions. Switching implementations now dominate applications in which bridging technologies were
implemented in prior network designs. Superior throughput performance, higher port density, lower
per-port cost, and greater flexibility have contributed to the emergence of switches as replacement
technology for bridges and as complements to routing te