For each additional switch you wish to stack, you will need a Cascade Extender Kit. The Cascade Extender Kit enables you to add one additional switch. The Cascade Extender Unit provides three ports, a blue down port, a yellow up port, and a modular port plugs directly into a Cascade module's D-type port enabling you to stack three or more switches together. It is a device that allows its host switch to be removed and replaced without disturbing the rest of the stack.