Basic port configuration

Speed and duplex

It can be useful to configure half-duplex if you’re connecting to some old tech, like a hub. For the user facing ports, you will probably prefer auto speed for these ports. However, autoconfiguration does not alway work within the infrastructure, and you may prefer to set link speed manually.

SW2(config)#int fa 1/0/30
SW2(config-if)#speed ?
10    Force 10 Mbps operation
100   Force 100 Mbps operation
auto  Enable AUTO speed configuration

SW2(config-if)#speed auto

SW2(config-if)#duplex ?
auto  Enable AUTO duplex configuration
full  Force full duplex operation
half  Force half-duplex operation

SW2(config-if)#duplex half

Another option worth knowing is MDIX. It’s the mechanism that allows the switch to know which twisted pair to use for transmission, and which for reception. It’s on auto by default, and can only be auto on my Catalyst 3750.

SW2(config-if)#mdix ?
auto  Enable automatic MDI crossover detection on this interface

SW2(config-if)#mdix auto

Get info on a specific interface

SW2#show int fa 1/0/30
FastEthernet1/0/30 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0015.2b2e.3c22 (bia 0015.2b2e.3c22)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
    reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, media type is 10/100BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:01, 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
    7041 packets input, 513625 bytes, 0 no buffer
    Received 5118 broadcasts (0 multicast)
    0 runts, 0 giants, 0 throttles
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
    0 watchdog, 5060 multicast, 0 pause input
    0 input packets with dribble condition detected
    2541 packets output, 256741 bytes, 0 underruns
    0 output errors, 0 collisions, 1 interface resets
    0 babbles, 0 late collision, 0 deferred
    0 lost carrier, 0 no carrier, 0 PAUSE output
    0 output buffer failures, 0 output buffers swapped out

This command also works with other interfaces like VLANs etc.