Network Interconnect Devices: Repeaters, Bridges, Switches, Routers

This page contains the following divisions:

• "Making The Connection"
  This is a poem by Joseph Bardwell that expresses a general thought about the confusion in the interconnect device marketplace.
• The Interconnect Box
  This discussion explains some of the general history of interconnect technology and lays the groundwork for a perspective on repeaters, bridges, switches, routers, and gateways.
• Interconnect Device Topics
  Here is the listing of the sub-topics in the section

"Making The Connection"

A poem by Joseph Bardwell

Sometimes it amazes me
that routers work at Layer 3
when switches very will could do
the job at simply Layer 2
But switches work at Layer 3
Oh, how confusing this can be
When bridges work at Layer 2
and routers can be bridges too!
And when you hope there'd be no more
you find a switch at Layer 4
So Layer 4, and 2, and 3
imply OSI conformity
But these are simply building blocks
in what we'll call an "Interconnect Box"

The Interconnect Box

Years ago, in the early days of desktop computing, an engineer would have learned about interconnect devices with some general differentiation's like these:

A repeater extends the allowable length of a cable   A bridge connects similar networks A router connects dissimilar networks

...but, bridges became more sophisticated. They could translate between Ethernet and Token-Ring networks and they supported multiple ports; not just two connections. Bridges were able to filter traffic on a selective basis through new configuration options.

...routers became more sophisticated. They supported much more than simple IP routing and they also had the ability to filter traffic on a selective basis. Something had to be done to clarify the definition of bridging versus routing. In the late 1980's and early 1990's we learned differentiation's like this:

The distinction between devices is based on their function in the OSI Model. A repeater operates at Layer 1 A bridge operates at Layer 2 A router operates at Layer 3

And now let's introduce the concept of switches! The switch takes the functions of the repeater and the bridge and combine them in clever ways to create a multi-port interconnect box that provides wonderful interconnectivity but challenges network protocol analyzer engineers. And let's complicate things by, essentially, merging the bridging and routing functions into a single box from Cisco, Bay Networks, 3Com, Thomflex, or other vendors.

That's why we can think of that 'box' in the wiring closet as, simply, "The Interconnect Box". Does the vendor call it a Router? Is it a "Layer Four Switch"? How about a "Brouter"; and what about the frame forwarding functionality that's inherent in a file server or a Unix box running the "routed" routing daemon?

This section of the compendium discusses the various forms and functions of many different types of interconnect box technologies that you may encounter. This section is by no means comprehensive - there's just too much information to present. We are covering the core technologies and we'll continue to update this section as we discover additional interesting ways that vendors discover to make forwarding decisions.

Topics In This Discussion

This section of the compendium includes the following sub-topics that discuss various elements of interconnection technology:

• Bridge Technology
  This is a general discussion of frame forwarding at Layer 2
• Switch Technology
  This is a general discussion of switching at Layer 2