Duplex and Speed settings are the most basic settings for each port of a switch. It is possible that the duplex and bandwidth settings between the switch port and the connected devices no match, just like a computer or another switch. There are two types of duplex settings used for communications on an Ethernet network. Full-duplex and Half-duplex, that we already discuss an earlier chapter.
Full-duplex
Both ends of the connection can send and receive simultaneously.
Half-duplex
Only one end of the connection can send at a time.
Auto-negotiation
Most Ethernet Switches has a function called auto-negotiation. This function is also available in NICs. Auto-negotiation makes possible two devices to automatically exchange information about duplex and speed settings. This function help switch and connected device to choose highest performance mode. If both devices have a capability full-duplex it will set both devices on full-duplex along with their highest common bandwidth.
See Figure 4.5, 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 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.
Duplex Mismatch
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 down the performance of devices.
Why duplex-mismatch occur?
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 4.6. 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 are reset, and the auto-negotiation process does not result in both link partners having the same configuration.
It also can occur 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.
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