speed mismatch

Duplex and Speed settings are the most basic settings for each switch port. By default, Cisco switches will auto-negotiate the speed and duplex settings. However, there is a possibility of duplex and speed settings mismatching between the switch port and the connected devices, such as a computer or another switch. Two types of duplex settings are used for communications on an Ethernet network. In full-duplex communication, both ends of the connection can send and receive simultaneously, but only one can be sent simultaneously in half-duplex communication.

Auto-negotiation

Most Cisco Switches have an auto-negotiation function; the NICs also contain these functions. Devices can auto-negotiate, automatically exchange information about duplex and speed settings, and help switch and connect devices to choose the highest performance mode. If both interconnected devices have a capability full-duplex, they will set both devices on full-duplex along with their highest expected bandwidth. See Figure below, where PC-1 is connected to switch port-1. Ethernet NIC and port can operate in full-duplex or half-duplex, so auto-negotiation sets 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 are connected with duplex settings that are not the same, either because of manual settings or the auto-negotiation process. Duplex Mismatch has also slowed down the device’s performance. 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 the Figure below.

Duplex mismatches occur when the Ethernet device or switch is hard-coded to full-duplex, and the other side is configured for auto-negotiation. When no auto-negotiation information is given, the switch defaults to half-duplex. This occurs when one or both ports on a link have reset, and the auto-negotiation 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.