Introduction to Layer 3 Switching

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.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.