Collisions in LAN Interface FastEthernet - CISCO troubleshooting

As part of my daily job as Network Operation Engineer, I received a case from a managed router that it's LAN Interface FastEthernet0/0 was keep on bouncing. Logs and interface stats was telling me that the interface was experiencing collisions. See details below.

millionaire# how log | inc FastEthernet0/0
Dec 23 22:51:23.554: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 22:51:24.974: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
Dec 23 22:52:03.170: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 22:52:05.170: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
Dec 23 22:52:16.421: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 22:52:18.421: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
Dec 23 22:54:00.919: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 22:54:02.918: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
Dec 23 22:54:22.300: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 22:54:24.300: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
Dec 23 22:54:30.624: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 22:54:32.623: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
Dec 23 22:57:32.806: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 22:57:34.806: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
Dec 23 23:00:11.174: %GT96K_FE-5-LATECOLL: Late Collision on int FastEthernet0/0
Dec 23 23:00:13.174: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up

rmillionaire# sho int FastEthernet0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is 001d.70a3.9c7a (bia 001d.70a3.9c7a)
Internet address is 10.4.144.1/24
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)
Half-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 1w2d
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 9000 bits/sec, 17 packets/sec
5 minute output rate 79000 bits/sec, 24 packets/sec
16045014 packets input, 1662718685 bytes
Received 2013833 broadcasts, 0 runts, 0 giants, 36 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog
0 input packets with dribble condition detected
32442539 packets output, 4191918373 bytes, 0 underruns
21612 output errors, 15059 collisions, 21595 interface resets <<-----
108454 unknown protocol drops
0 babbles, 21639 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out

Initial investigation was telling me that the LAN interface FastEthernet0/0 was configured as speed and duplex auto. But I found out that the interface resulted to be in half duplex 100 Mbps. By theory, it is telling me that the device or equipment that is connected to this interface is configured also to 100 Mbps Half Duplex but I have no way to verify it because we don't manage that equipment. In order to know this, I should call the customer to do it for me which is not appropriate at this time because we have time zone difference of 13 hours. See, if there is a sunlight here where I am standing, I'm sure, they are now sleeping or out of office already. Who want to be called out business hour? I don't either.

millionaire# sho int FastEthernet0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is 001d.70a3.9c7a (bia 001d.70a3.9c7a)
Internet address is 10.4.144.1/24
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)
Half-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 1w2d
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 9000 bits/sec, 17 packets/sec
5 minute output rate 79000 bits/sec, 24 packets/sec


So I tried my luck, and googling it to find the answer. I found out that some routers have issue in their auto-duplex and speed config. The possible work around is to force the interface config to 100 Mbps full duplex and boom, it solved the problem. Collisions disappeared in the interface and still 0 errors after clearing the counters.

millionaire# show running-config interface FastEthernet0/0
Building configuration...

Current configuration : 144 bytes
!
interface FastEthernet0/0
ip address 172.30.20.9 255.255.255.252 secondary
ip address 10.4.144.1 255.255.255.0
speed 100
full-duplex
end

router# show interfaces FastEthernet0/0
FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is 001d.70a3.9c7a (bia 001d.70a3.9c7a)
Internet address is 10.4.144.1/24
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 00:00:37
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 5000 bits/sec, 10 packets/sec
5 minute output rate 346000 bits/sec, 31 packets/sec
463 packets input, 32677 bytes
Received 101 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
400 packets output, 32990 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
5 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


END --->