Cisco CRC Errors

Posted on 2007-04-09
Last Modified: 2008-01-09
Cisco 2801 CRC errors.  We have had errors on our FA0/0 interface.  What should I look for with these errors.  The interface is set to full 100mMb, 100BaseTx/FX.   What does the 100BaseTX/FX from the show interface refer to?  I am trying to figure out if we have a port mismatch,  possibly a device sending bad packets, etc.  Thanks for the help
Question by:mkurtzhals
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LVL 79

Accepted Solution

lrmoore earned 500 total points
ID: 18878811
CRC errors could mean bad cable or duplex mismatch.
Make sure the speed/duplex matches whatever is plugged into that interface. I'm assuming it is plugged into a switch? Is the switchport also hard-set to 100/full or is it auto? They both have to be the same - 100/full or auto
Do you get collisions on this link also? If yes, then duplex mismatch for sure.
Is the patch cable tested/certified CAT5? Homemade?

> What does the 100BaseTX/FX from the show interface refer to?
It just means it is a standard 100Mb Ethernet interface.

Here is a sample output from my own 2801. I know that I do not have a duplex mismatch, but I am connected to a Nortel switch and not a Cisco switch. The Nortel switch was having issues and was replaced last week.
Some CRC errors, but not many compared to the total number of packets.

 5 minute output rate 2000 bits/sec, 2 packets/sec
    13611873 packets input, 3056715987 bytes
    Received 13504802 broadcasts, 52 runts, 0 giants, 0 throttles
    201 input errors, 201 CRC, 2 frame, 0 overrun, 0 ignored
    0 watchdog
    0 input packets with dribble condition detected
    1062902 packets output, 124156282 bytes, 0 underruns
    0 output errors, 0 collisions, 8 interface resets
LVL 28

Expert Comment

ID: 18878819
There is an output interpreter on Cisco's website that you can use to paste the output of the "show interface" command.  Here's the link:

However, it requires a Cisco login to access it.  It may be a good idea for you to paste the output of the "show interface fa0/0" command anyway so the community can take a look at the specific errors you are getting...

Author Comment

ID: 18879104
The port on the switch was set to auto.  It negotiatd 100 half for some reason.  Hard-coded in 100 full and no more errors!
LVL 79

Expert Comment

ID: 18881681
Here's the deal....
The router was set to 100/full
The switch was set to auto
With the router set to something other than auto - like 100/full - you disable its ability to negotiate with the switch, when you plug it into the switch, the switch will try to negotiate. The router does not respond because it is already set. No response from the end device to the switch means that the switch will default to thinking that the only devices that don't auto negotiate are dumb hubs and hubs are always half-duplex. So the default is to fall back to half-duplex anytime the end device does not respond to the negotiation.
Any time you hard-set speed/duplex on any end device, that 'turns off' the negotiation. No negotiate=half-duplex. Everything is working "as designed".

Author Comment

ID: 18882997
THANKS for your help lrmoore.  Helped me plenty of times,  Keep it up.

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