Duplex and Speed settings are the most basic settings for each port of a switch. By default, Cisco switches will auto-negotiate the speed and duplex settings. There is a possibility of mismatching of duplex and speed setting between the switch port and the connected devices, for example, a computer or another switch. There are two types of duplex settings used for communications on an Ethernet network. Full-duplex communication both ends of the connection can send and receive simultaneously but in the half-duplex, only one end of the connection can send at a time.
Most of Cisco Switches have an auto-negotiation function and the NICs has also contained these functions. Devices have the capability of auto-negotiation can automatically exchange information about duplex and speed settings and helps switch and connected device to choose the highest performance mode. If both interconnected devices have a capability full-duplex; they well set both devices on full-duplex along with their highest common bandwidth. See Figure below, where PC-1, is connected to switch port-1. Both Ethernet NIC and port can operate in full-duplex or half-duplex, so auto-negotiation set both sides on full-duplex.
The speed of switch is 10/100/1000 Mb/s and PC-1 Ethernet Speed is 10/100 Mb/s so 100; is the highest common speed for both switch and PC-1, therefore, auto-negotiation set speed for 100 Mb/s for both. Most Cisco switches and Ethernet NICs default to auto-negotiation for speed and duplex. Gigabit Ethernet ports only operate in full-duplex mode. The figure below illustrates the auto-negotiation for speed settings between switch and PC-1.
Duplex Mismatch and speed settings
A duplex mismatch occurs when the two communicating Ethernet devices connected with duplex settings; that are not the same, either because of manual settings or the auto-negotiation process. Duplex Mismatch has also down the performance of the device. An example of a duplex mismatch is if one port on the link operates; at half-duplex while the other port operates at full-duplex, as shown in Figure below.
Duplex mismatches occur when either the Ethernet device; or the Ethernet switch is hard-coded to full-duplex and the other side is configured for auto-negotiation. This happens because the switch, when not given any auto-negotiation information, will default to half duplex. This occurs when one or both ports on a link have reset and the auto-negotiation; the process does not result in both link partners having the same configuration.
It also occurs when users reconfigure one side of a link and forget to reconfigure the other. Both sides of a link should have auto-negotiation on, or both sides should have it off.