Cisco catalyst an CRC errors

[victor gomes do vale]

Hello all,

I've been searching for a strange problem on some offices I manage, and cannot find any answer on this one:

Some users were experiencing slow network connection, the network ports on the switchs (Catalysts 2960G) are usually auto and they get negociated at 1000/full.

when checking the port status the port indicates that no input error nor CRC were detected.

Then I force the speed to 100 and duplex full, and then the port reports input errors and CRCs on the port.

The solution was simple, just changed the cable and/or the wall network plug, and the problem was gone (user back to gigabit).

My question is: Why do I need to speed down to 100/full, to see the CRC errors ? since latency and network was really slow there was errors (and doing a wireshark on the main gateway showed me packet losses) so why does no CRC appears on speed 1000 ?

Thanks a lot for your help and ideas,

[Ziga M]

If you leave autonegotiations, then link will be 1000/full. But if you change only on one side (on switch), then PCs will 'fall back' to half dupex. Try to set to full duplex on PC too and shouldnt see CRCs. 

[victor gomes do vale]

Let me clarify your explanation, do you mean that since it is 1000/full auto on the switch, the client falls back to "half duplex" without that the switch is aware of ??

My question is NOT how can i get rid of CRCs, since i know it comes from bad cable, that i replaced and corrected the CRC problem.

My question IS why do CRC APPEARS in stats in 100/full and NOT IN 1000/full ? because on both link types cables WAS bad and network was faulty/slow !

[Ziga M]

hmm, I'm not 100%, but I think you can't have 1000/half. Meaning when you force to 1000/full, the other side will set the same setting (1000/full). But when you force to 100/full, other side will set 100/half, causing CRCs.

[victor gomes do vale]

Hi again,

You're answer still confuse me, don't know if its me that is not clear enought.

Let me try to explain it again :

- 1000/full was AUTO (i do NOT force) switch set it automatically

- Networks was bad (laggy/slow) on both 100/full and 1000/full, BUT the switch only reports the CRCs on 100/full. and DO NOT report them on 1000/full, even if the network WAS laggy/slow on 1000/full (meaning errors on the link! but not reported by switch)

- Changing the cable DO have corrected my problem (no more CRC on 100/full AND 1000/full)

My question IS : Why is the switch reporting CRCs only in 100/full and not 1000/full, since network problems were happening on both link types ?

[Leo Laohoo]

Why do I need to speed down to 100/full, to see the CRC errors ? since latency and network was really slow there was errors (and doing a wireshark on the main gateway showed me packet losses) so why does no CRC appears on speed 1000 ?

Not all NIC cards drivers are mostly to blame.  Make sure your NIC cards have the latest drivers.  The drivers must be sourced from the manufacturer of the NIC card and not from the manufacturer of the PC.  

Exactly what command was used to set the speed?  Did you use the command "speed auto 100" or "speed 100"?   

The command "speed 100" basically means TURN OFF AUTO-NEGOTIATION and set speed to 100.  The former means "auto-negotiate speed for 100".  Two different command and will give you two different results.  

[David_Che · Accepted solution]

switch have two switching mode: (1) cut-through (2) store-and -forward.

In cut-through mode, switch only check the ethernet frame header 64 bytes (always), then switching to outgoing port, even there are  CRC error in this frame. However this switch Do know this is CRC-error frame, it will increase one CRC error in the outgoing port, in outbound direction.

In store-and-forward mode, switch will check all the full frame, it will discard this faulty frame if CRC error, and increment one CRC error in incoming port , inbound direction.

I guess 100/full works at store-and -foward mode,  1000/full works at cut-through mode.

Hope it woks.

Regards,

David

[victor gomes do vale]

Hi,

Thanks a lot for this info, this really can explain the problem and i was not aware of that cut-throught/store-and-forward thing.

Thanks again.

Victor