During your career in internetworking, you will continuously hear talk of equipment functioning at different “layers”. As a recurring theme since Chapter 1, you should now be aware that the layers being referred to are those in the OSI model. While the concept of a switch or a bridge as a Layer 2 device or a router as a Layer 3 device should now seem elementary, it’s easy to become confused by the mish-mash of marketing lingo that pervades the industry. What is a Layer 4 switch, for example? Well, it depends on whom you ask. Ultimately, vendors tend to use the different layers of the OSI model to represent intelligent decision-making features in their equipment. In some cases, as with Layer 3 switching, the term used represents a clearly defined and valid function. In others, like Layer 4 switching, what the term actually means can be a little less clear.
At the most basic level, the role of a Layer 3 switch is more or less identical to that of a router. Recall that a Layer 2 switch makes forwarding decisions based of the destination MAC address of a frame. In the same way, a Layer 3 switch is also capable of carrying out the functions of a router, making forwarding decisions based on the destination IP address of a packet. For all intents and purposes, a Layer 3 switch is basically a traditional Layer 2 switch that is also capable routing traffic between different subnets or networks. The big difference with a Layer 3 switch is usually speed, namely the speed at which it is capable of routing. Recall that Layer 2 switching is typically a much faster operation than routing, if only because there is less work involved in the forwarding process. With a Layer 3 switch, routing can often occur at close to the same forwarding rates as those associated with Layer 2 switching.