Bridge Basics

One common cause of confusion in the networking world is the difference between a bridge and a router, and the difference in the expected behavior of each. This essay will outline the reasons bridges were created, define the expected behavior of a bridge, and finally, will close with a concise definition of a bridge.

For a much more detailed treatment of the material in these essays WildPackets recommends Radia Perlman's book "Interconnections - Bridges and Routers," which is available from Addison-Wesley, ISBN number 0-201-56332-0.

Why Use Bridges?

As anyone with networking experience knows, segmenting a large network with an interconnect device has numerous benefits. Among these are reduced collisions (in an Ethernet network), contained bandwidth utilization, and the ability to filter out unwanted packets. However, if the addition of the interconnect device required extensive reconfiguration of stations, the benefits of the device would swiftly be outweighed by the administrative overhead required to keep the network running. Bridges were created to allow network administrators to segment their networks transparently. What this means is that individual stations need not know whether there is a bridge separating them or not. It is up to the bridge to make sure that packets get properly forwarded to their destinations. This is the fundamental principle underlying all of the bridging behaviors we will discuss.

It should be noted that IBM invented a second type of non-transparent bridge, called a source-routing bridge. These are emphatically not the types of bridges we're discussing here.

  • Transparent Bridges: A bridge is said to be "transparent" because a transmitting station addresses frames directly to the DLC address of the destination station. This section discusses this concept of "transparency".
  • Learning Bridges: This section describes the technology used in Learning Bridges. The alternative to a Learning Bridge is a Source Route Bridge.
  • Spanning Tree Algorithm: A set of rules for determining bridge paths, called the 'Spanning Tree' algorithm, is used to prevent bridge loops in networks. This section discusses the implementation of the Spanning Tree Algorithm.
  • Bridge Loops: If a bridge loop occurs in a network the results are often unpredictable but often catastrophic. This section overviews the issues related to bridge loops.

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