Cisco 3560 CRC errors and Dribbling | Easy Fix Bad Patch Cable

I ran across a strange issue that took entirely way too long to figure out.  I was doing a routine IT support visit when I noticed one buildings locations had less then optimal speeds.   The download speeds were hovering around 12-40 mbps when typically our connection runs a strong 190-350 mbps (depending on wifi or wired).  At first I thought was perhaps just a temporary issue with congestion but after checking throughout the day I found that no it was isolated to one building (Unit Office) and only affecting traffic here.

 

For good measure I updated the Firmware on our wireless controller and even updated the switches iOS since it had not been done in some time.

I have two switches in this location one a Catalyst 2560G and a Catalyst 3560.¬†¬† Looking thought the switches configuration I could not find anything that stood out.¬† No loopbacks…no faulty nics on the client side.¬†¬† Upon closer inspection (show interface command) I found that the tengigabitethernet0/2 interface was showing CRC errors and dribble conditions detected.¬†¬†¬† Here after comparing configurations checking for auto negiotiation issues I could not find anything wrong.¬†¬† After clearing the counters and bringing the port down and back up CRC errors continued.

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A long story short after reading and re-reading cisco documentation on everything from packet flooding, ARP timeout mismatches, QOS settings, etc I stumbled across the answer from a Cisco guy posting on a forum.  It simply said in his experience CRC errors almost always indicate a bad cable or sometimes a bad SFP or similar module.

 

The Cable was bad.¬†¬†¬† That’s it.

 

Yes Inside the fiber panel the SC Fiber patch cable had been bent inside the cabinet in a such a way that the cable made a hard 90 degree bend.  Simply replacing this cable and everything went immediately back to normal.  After clearing the counters on the switch after 60 minutes 0 CRC errors were detected.    The lesson for me is before looking and spending hours reading complex networking topology issues probably is a lot smarter to start simple, check cables and SFP modules.