Chapter 3. Buffers

When you begin talking to vendors about data center switches, you’ll hear and read about buffers. Some of the vendors have knockdown, drag-out fights about these buffers, and often engage in all sorts of half-truths and deceptions to make you believe that their solution is the best. So, what is the truth? As with most things, it’s not always black and white.

To begin, we need to look at the way a switch is built. That starts with the switch fabric.


The term fabric is used because the interconnecting lines of ports and switches evokes the weave of a fabric viewed through a microscope. And all this time I thought there was some cool scientific reason.

Imagine a matrix in which every port on the switch has a connection for input (ingress) and another for output (egress). If we put all of the ingress ports on the left and all the output ports on top and then interconnect them all, it would look like the drawing in Figure 3-1. To make the examples easy to understand, I’ve constructed a simple, though thoroughly unlikely, three-port switch. The ports are numbered ethernet1, ethernet2, and ethernet3, which are abbreviated e1, e2, and e3.

Looking at the drawing, remember that e1 on the left and e1 on the top are the same port. This is very important to understand before moving forward. Remember that modern switch ports are generally full duplex. The drawing simply shows the ins on the left and the outs on the top. Got it? Good. Let’s continue.

First, the fabric ...

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