Switch ports can be manually configured with specific duplex and speed settings. The respective interface configuration commands are duplex and speed
The default setting for both duplex and speed for switch ports on Cisco Catalyst 2960 and 3560 switches is auto
Mismatched settings for the duplex mode and speed of switch ports can cause connectivity issues. Autonegotiation optimizes Duplex and Speed settings
Autonegotiation is useful when the speed and duplex settings of the device connecting to the port are unknown or may change