Now that you’re familiar with Layer 3 switching, you’re probably curious about what Layer 4 switching represents. Well, the answer isn’t as difficult as you might have imagined. Quite simply, a Layer 4 switch is typically just a Layer 3 switch that is also capable of making decisions based on Layer 4 information. Layer 4 (the Transport Layer) carries information about the source and destination TCP and UDP ports in use, which generally represent unique applications. Because of this, a Layer 4 switch is capable of making forwarding decisions according to the applications in use.
For example, an administrator might choose to prioritize VoIP traffic through the use of Quality of Service (QoS) features, granting VoIP applications more bandwidth. Conversely, the Layer 4 port information could also be used to route the packets from certain applications along a different path than other traffic. Ultimately, a Layer 4 switch gives administrators a higher level of control over how bandwidth is used within a network.