Though transparent to network protocols and user applications, switches can operate in different modes that can have both positive and negative effects when forwarding Ethernet frames on a network. One of the most basic settings of a switch is the duplex setting of each individual port connected to each host device. A port on a switch must be configured to match the duplex settings of the media type. There are two types of duplex settings used for communications on an Ethernet network: half duplex and full duplex.

Half Duplex

Half-duplex communication relies on unidirectional data flow where sending and receiving data are not performed at the same time. This is similar to how walkie-talkies or two-way radios function in that only one person can talk at any one time. If someone talks while someone else is already speaking, a collision occurs. As a result, half-duplex communication implements CSMA/CD to help reduce the potential for collisions and detect them when they do happen. Half-duplex communications have performance issues due to the constant waiting, because data can only flow in one direction at a time. Half-duplex connections are typically seen in older hardware, such as hubs. Nodes that are attached to hubs that share their connection to a switch port must operate in half-duplex mode because the end computers must be able to detect collisions. Nodes can operate in a half-duplex mode if the NIC card cannot be configured for full duplex operations. In this case the port on the switch defaults to a half-duplex mode as well. Because of these limitations, full-duplex communication has replaced half duplex in more current hardware.

Full Duplex

In full-duplex communication, data flow is bidirectional, so data can be sent and received at the same time. The bidirectional support enhances performance by reducing the wait time between transmissions. Most Ethernet, Fast Ethernet, and Gigabit Ethernet NICs sold today offer full-duplex capability. In full-duplex mode, the collision detect circuit is disabled. Frames sent by the two connected end nodes cannot collide because the end nodes use two separate circuits in the network cable. Each full-duplex connection uses only one port. Full-duplex connections require a switch that supports full duplex or a direct connection between two nodes that each support full duplex. Nodes that are directly attached to a dedicated switch port with NICs that support full duplex should be connected to switch ports that are configured to operate in full-duplex mode.

The figure shows the two duplex settings available on modern network equipment.

A Cisco Catalyst switch supports three duplex settings:

For Fast Ethernet and 10/100/1000 ports, the default is auto. For 100BASE-FX ports, the default is full. The 10/100/1000 ports operate in either half- or full-duplex mode when they are set to 10 or 100 Mb/s, but when set to 1,000 Mb/s, they operate only in full-duplex mode.