duplex [full|half|auto]
Sets the duplex mode on a switch or router interface to full, half, or auto-negotiation.
duplex [full|half|auto]When to Use This Command
- Configuring a server port to full-duplex for maximum throughput.
- Setting a legacy hub-connected port to half-duplex to avoid collisions.
- Enabling auto-negotiation on a port connecting to an unknown device.
- Troubleshooting duplex mismatch by manually setting both ends to the same duplex.
Command Examples
Set interface to full-duplex
interface GigabitEthernet0/1
duplex fullNo output is displayed when the command is entered. Use 'show interfaces GigabitEthernet0/1' to verify.
Verify duplex setting after configuration
show interfaces GigabitEthernet0/1GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is 0011.2233.4455 (bia 0011.2233.4455)
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is RJ45
output flow-control is unsupported, input flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
0 unknown protocol drops
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier, 0 pause output
0 output buffer failures, 0 output buffers swapped outThe line 'Full-duplex, 1000Mb/s' confirms the interface is operating in full-duplex mode at 1 Gbps. Other fields show interface status, errors, and traffic statistics.
Understanding the Output
The 'show interfaces' command displays the current duplex setting on the line showing duplex and speed (e.g., 'Full-duplex, 1000Mb/s'). A duplex mismatch is indicated by high numbers of collisions, late collisions, or CRC errors. For half-duplex, you would see 'Half-duplex'. Auto-negotiation is shown as 'Auto-negotiation' or the negotiated duplex. Always verify both ends match to avoid performance issues.
CCNA Exam Tips
CCNA exam expects you to know that duplex mismatch causes collisions and poor performance.
Remember that half-duplex is used with hubs; full-duplex is used with switches.
Auto-negotiation is the default on most Cisco interfaces; manual settings should match on both ends.
The 'duplex' command is available in interface configuration mode; use 'show interfaces' to verify.
Common Mistakes
Setting one end to full-duplex and the other to half-duplex, causing duplex mismatch and packet loss.
Forgetting that auto-negotiation must be enabled on both ends for it to work; manually setting duplex disables auto-negotiation.
Using 'duplex half' on a switch port connected to a server that expects full-duplex, reducing throughput.
Related Commands
interface [type] [number]
Enters interface configuration mode for a specific interface (e.g., GigabitEthernet0/1) to configure Layer 2 or Layer 3 parameters.
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
speed [10|100|1000|auto]
Sets the speed of a Cisco switch or router interface to a specific value or auto-negotiation.
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