show int fa0/3 commands

Answered Question
Jun 25th, 2009
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Hi every body!


I have few questions:


1) please consider a full duplex enviroment.


If i use the command :


show int fa0/3


it will show a lot of informations. My focus is CRC counter.


CRC counter increments if frame fails

fcs.But what are the possible reasons that may cause CRC counter to increment in full duplex enviroment?



Here is what my book says:


"Regardless of root cause, whenever the electrical signals degrades, the receiving device may receive a frame with whose bits have changed value. These frames do not pass the error detection logic as implemented in the fcs field in the ethernet."



Based on book, the sole reason that can cause frame to fail fcs is signal degradtion.

Signal degradition can be caused by inappropriate wiring, that is not using the necessary cabling( wrong category of utp),Emi from outside interference.


Is there any thing else besides signal degradtion that can cause frame to fail fcs test?


2) please consider half duplex enviroment.


Again the same command" show int fa0/3" will show a lot of counters.

My focus are CRC , collision and late collision counters.

In such enviroment, do collisions on half duplex ethernet have any bearing on CRC counter? To put it another way, is it possible for a switch to receive a frame which has experiences a collision but yet passes the fcs test?


thanks a lot!


Correct Answer by glen.grant about 8 years 1 month ago

You can also use "show interface counters errors " to look at port errors. If you have a full duplex on one end and half on the other this will increment the crc counter also , have seen it many times.

Correct Answer by Leo Laohoo about 8 years 1 month ago

Hi Zeeshan,


Depending on the model of the switch, I'm not a big fan of the "sh interface " to look at the input/output errors. The "sh controller ethernet " gives you a detailed view particularly what types of input/output errors.


Hope this helps.

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Correct Answer
Leo Laohoo Thu, 06/25/2009 - 19:56
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Hi Zeeshan,


Depending on the model of the switch, I'm not a big fan of the "sh interface " to look at the input/output errors. The "sh controller ethernet " gives you a detailed view particularly what types of input/output errors.


Hope this helps.

Leo Laohoo Thu, 06/25/2009 - 20:37
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Thanks for the ratings, Zeeshan.

Correct Answer
glen.grant Fri, 06/26/2009 - 04:04
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You can also use "show interface counters errors " to look at port errors. If you have a full duplex on one end and half on the other this will increment the crc counter also , have seen it many times.

viyuan700 Fri, 06/26/2009 - 08:26
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is there any thing else besides signal degradtion that can cause frame to fail fcs test?


possible causes of data-link errors when configured for full-duplex are bad cables, faulty switch ports or Network Interface Card (NIC) software or hardware issues.



To put it another way, is it possible for a switch to receive a frame which has experiences a collision but yet passes the fcs test?


I dont think it is possible to pass FCS as per CSMA/CD if collision is detected transmitter stop sending and a jam signal is sent to inform other stations to not send data.



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