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Community Member


I'm seeing collisions on my switch port. P/C is connecting at 100Mbps Full , but i'm still seeing collisions. Never seen this before. Could it be cabling?

Cisco Employee

Re: Collisions

Hi Friend,

Can you paste the output of "sh interface " on which this pc is connected? Also what is the cable distance from pc to switch?



Community Member

Re: Collisions

The cable distance is about 100ft.

RD-IDF2-SWITCH1>show int fa0/39

FastEthernet0/39 is up, line protocol is up (connected)

Hardware is Fast Ethernet, address is 0012.4308.eb27 (bia 0012.4308.eb27)

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 100BaseTX

input flow-control is off, output flow-control is unsupported

ARP type: ARPA, ARP Timeout 04:00:00

Last input 1y47w, output 00:00:01, 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 32000 bits/sec, 29 packets/sec

5 minute output rate 65000 bits/sec, 43 packets/sec

219928108 packets input, 2426567361 bytes, 0 no buffer

Received 579845 broadcasts (0 multicast)

0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog, 206 multicast, 0 pause input

0 input packets with dribble condition detected

1387771339 packets output, 2538516768 bytes, 0 underruns

0 output errors, 32881 collisions, 1 interface resets

0 babbles, 0 late collision, 628 deferred

0 lost carrier, 0 no carrier, 0 PAUSE output

0 output buffer failures, 0 output buffers swapped out

Re: Collisions

Sorry, scrap that last conmment - I wrote it before I saw your show int.

The fact is, there is no such thing as a collision in full-duplex. Could this collision count be from a former connection to this port that was operating in half-duplex? Or perhaps with the PC hard-coded and the switchport in auto?

If you clear down the counters now, do the collisions still count up?

Do you have any evidence that the autonegotiation might fail sometimes due to the long cable length. If so, you might do well to hard-code both the PC and the switchport.

Kevin Dorrell


Re: Collisions

If the PC is configured 100 full-duplex, adn the switch is in auto, then the switch will actually operate in half duplex, and you will see collisions ... lots and lots of them.

The reason is that if you hard-code the PC into 100-full, then you switch off the negotiation. The switch, not seeing any negotiation, reverts to half-duplex. Result, collisions.

If you hard-code the PC you should hard-code the switchport. If the PC is auto-negotiating, then the switchport should be as well. Do not mix the two.

Kevin Dorrell


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