Chapter 5. Fabric Speed

One of the things you’ll hear over and over from switch vendors is how fast their switch is, how fast the fabric is, or how much backplane capacity it has. But what does all that mean?


This chapter is not intended to explain every possible detail of switch fabrics, but rather to help you understand what the term fabric speed means in a general sense. Entire books could be written on this topic, but my goal here is to help you understand the confusing and often misleading numbers that many vendors include in their switch specification sheets.

In a top-of-rack (ToR) switch, the fabric is the interconnections between all the interfaces. The term backplane in this case is pretty much synonymous with fabric (though probably inaccurate). On a chassis switch, the terms can be thought of a bit differently.

On a chassis switch, each module can have a fabric, and interfaces within a module can switch between each other while staying local to the blade. When a packet sourced on one blade must travel to another blade, though, the packet needs a path between the blades. The connections between the blades are often called the backplane, although really, that term is more about the hardware connecting the modules to one another. Semantics aside, what you need to know is that there is a master fabric connecting all these modules together in a chassis switch.

In modern chassis switches, like the Arista 7500R and Cisco Nexus chassis models, the backplane fabric resides ...

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