When selecting a switch, it is important to understand the key features of the switch options available. This means that it is necessary to decide on features such as whether Power over Ethernet (PoE) is necessary, and the preferred "forwarding rate".
As shown in Figure 1, PoE allows a switch to deliver power to a device, such as IP phones and some wireless access points, over the existing Ethernet cabling. This allows more flexibility for installation.
The forwarding rate defines the processing capabilities of a switch by rating how much data the switch can process per second. Switch product lines are classified by forwarding rates. Entry-layer switches have lower forwarding rates than enterprise-layer switches. Other considerations include whether the device is stackable or non-stackable as well as the thickness of the switch (expressed in number of rack units), and port density, or the number of ports available on a single switch. The port density of a device can vary depending on whether the device is a fixed configuration device or a modular device.
These options are sometimes referred to as switch form factors.
Fixed Configuration Switches
Fixed configuration switches are just as you might expect, fixed in their configuration. What that means is that you cannot add features or options to the switch beyond those that originally came with the switch. The particular model you purchase determines the features and options available. For example, if you purchase a 24-port gigabit fixed switch, you cannot add additional ports when you need them. There are typically different configuration choices that vary in how many and what types of ports are included.
Modular switches offer more flexibility in their configuration. Modular switches typically come with different sized chassis that allow for the installation of different numbers of modular line cards. The line cards actually contain the ports. The line card fits into the switch chassis like expansion cards fit into a PC. The larger the chassis, the more modules it can support. As you can see in the figure, there can be many different chassis sizes to choose from. If you bought a modular switch with a 24-port line card, you could easily add an additional 24 port line card, to bring the total number of ports up to 48.
Figure 2 displays examples of fixed configuration, modular, and stackable configuration switches.