On Cisco Devices the interface speed and duplex is set to auto by default. This means that both sides of a link should be able ton auto-negotiate to full duplex and the fastest available speed. However, it is best practice to manually set the speed and duplex of ports connected to another network infrastructure device or server - and in particular, it is critical to set matching speeds and duplex settings on both sides of the link.
Note
The practice of setting manual speeds applies to our own infrastructure devices - that is, switches, routers, firewalls, etc. We do not need to manually set speeds of other individual hosts within the network.
Important
When setting speed and duplex, ensure that both sides have the same setting: either full auto, or the same speed and duplex setting.
Setting speed and duplex
The duplex options are either full or half, while the speed options are set to match the slowest interface in the link. For example, here we have a “FastEthernet” interface; thus, it has a speed of 100 mbps.
SW1(config)# inteface FastEthernet 0/1
SW1(config-if)# duplex full
SW1(config-if)# speed 100