2.3 Troubleshoot interface and cable issues (collisions, errors, duplex, speed)

Switches eliminate collisions by creating a collision domain for each interface when the interface runs in full duplex. Full duplex means that the switch and the device can simultaneous send and receive – this means no collisions can occur.
However if the switch is running in half duplex collisions can occur because only one device can communicate at a time – similar to normal verbal communication.
Packet collisions are rare in modern networks. Back before the invention of the switch, hubs were used. Hubs are simply repeaters – they receive electrical signals (Layer 1) and repeat them. This means anything connected to a hub is in the same collision domain. This is similar to having a conversation within earshot of someone. If multiple people try to talk in the vicinity of each other the information gets jumbled.
The invention of the switch meant this problem was eliminate meaning more than one device could use the network at a time. This dramatically increases performance.
You can check for collisions by issues the “show int” command:

R1#show int f0/0
FastEthernet0/0 is up, line protocol is up
 Hardware is Gt96k FE, address is c201.1352.0000 (bia c201.1352.0000)
 Internet address is
 MTU 1500 bytes, BW 100000 Kbit/sec, 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, 100BaseTX/FX
 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: 1/75/0/0 (size/max/drops/flushes); Total output drops: 0
 Queueing strategy: fifo
 Output queue: 0/40 (size/max)
 5 minute input rate 69000 bits/sec, 23 packets/sec
 5 minute output rate 69000 bits/sec, 23 packets/sec
 10702 packets input, 5614386 bytes
 Received 6 broadcasts, 0 runts, 0 giants, 0 throttles
 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
 0 watchdog
 0 input packets with dribble condition detected
 10734 packets output, 5616917 bytes, 0 underruns
 0 output errors, 123 collisions, 4 interface resets
 0 unknown protocol drops
 0 babbles, 0 late collision, 0 deferred
 0 lost carrier, 0 no carrier
 0 output buffer failures, 0 output buffers swapped out

Collisions indicate a speed and or duplex mismatch. Here one interface is 10Mbps and the other is 100Mbps causing collisions.
When troubleshooting connectivity issues make sure the speed and duplex match on both ends.

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