Load-balanced Switch - Introduction

Introduction

Internet routers are typically built of line cards connected with a switch. Routers supporting moderate total bandwidth may use a bus as their switch, but high bandwidth routers typically use some sort of crossbar interconnection. In a crossbar, each output connects to one input, so that information can flow through every output simultaneously. Crossbars used for packet switching are typically reconfigured tens of millions of times per second. The schedule of these configurations is determined by a central arbiter, e.g. a Wavefront arbiter, in response to requests by the line cards to send information to one another.

Perfect arbitration would result in throughput limited only by the maximum throughput of each crossbar input or output. For example, if all traffic coming into line cards A and B is destined for line card C, then the maximum traffic that cards A and B can process together is limited by C. Perfect arbitration has been shown to require massive amounts of computation, that scales up much faster than the number of ports on the crossbar. Practical systems use imperfect arbitration heuristics (e.g. iSLIP) that can be computed in reasonable amounts of time.

A load-balanced switch is not related to a load balancing switch, which refers to a kind of router used as a front end to a farm of web servers to spread requests to a single website across many servers.