How do you manually configure speed and duplex on a Cisco switch port?

Use the duplex and speed interface-configuration commands (both default to auto).

S1# configure terminal
S1(config)# interface FastEthernet 0/1
S1(config-if)# duplex full
S1(config-if)# speed 100
S1(config-if)# end
S1# copy running-config startup-config

Default settings: Both duplex and speed default to auto on Catalyst 2960/3560 switches.

When to use autonegotiation (auto):

• Device connecting to the port is unknown or may change
• End-user workstations with varying capabilities

When to manually set speed/duplex:

• Connecting to known devices (servers, dedicated workstations, network devices)
• Best practice for server and infrastructure links

Important caveats:

• 10/100 ports can operate in half or full-duplex at 10 or 100 Mbps
• At 1000 Mbps (Gigabit), ports operate only in full-duplex
• Mismatched settings cause connectivity issues — always verify both ends match
• Fiber-optic ports (e.g., 1000BASE-SX) operate at one preset speed and are always full-duplex

Go deeper:

• Autonegotiation (Wikipedia) — the auto default you override and why a fixed/auto pairing causes a duplex mismatch.
• Switch Speeds and Forwarding Methods (Cisco Press) — the CCNA companion page covering duplex/speed and Auto-MDIX on Catalyst switches.