The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

17.3. Switch Fabrics

The Receive data path performs all of the necessary qualification, classifica-tion, and table lookup functions needed to determine to which output port(s) a given frame should be forwarded. The Transmit data path takes these forwarded frames, applies additional qualifiers as needed, implements any prioritization and distribution policies, and sends the frames onto the appropriate output(s). A switch fabric sits between the Receive and Transmit data paths; its function is to transfer frames among all of the input and output ports of the switch. The design of the internal switch fabric is critical to the performance of a switch. While a comprehensive treatment of this subject could fill its own book, in this section we look at three switch fabric architectures that have been widely used in commercial LAN switch products:

• Shared memory

• Shared bus

• Crosspoint matrix

Each approach reflects a distinct method used to move frames from input to output ports, and has its own set of characteristics, limitations, and design issues.

17.3.1. Shared Memory

A shared memory architecture entails the lowest level of both design and implementation complexity, and usually provides the lowest-cost solution in those products where it can be used. As a result, it is the most popular approach to fabric design in LAN switches. The primary limitation of a shared memory switch fabric is the memory bandwidth; the speed at which data can be moved into and out of the shared memory will place ...