On shared Ethernet, CRC errors are often the result of a collision. Let's leave that aside, however, and assume that you are referring to CRC errors on full-duplex Ethernet or serial links. CRC errors are caused by noise, signal reflections, impedance mismatches, improperly installed demarcs, faulty hardware, and other bad things that really shouldn't happen. The number should be really low. That's helpful, eh? :-)
CRC errors should be less on fiber-optic cabling compared to copper cabling. According to industry standards, fiber-optic cabling should not experience more than one bit error per 10^11 bits. Copper cabling should not experience more than one bit error per 10^6 bits. Some documents from Cisco and other vendors specify a threshold of one bad frame per megabyte of data. In other words, an interface should not experience more than one CRC error per megabyte of data received. (The "megabyte of data" threshold comes from the industry standards that state that copper cables should not have a bit error rate that exceeds 1 in 10^6.) This method is better than simply calculating a percentage of bad frames compared to good frames, which does not account for the variable size of frames. (If you have a constant flow of 64-byte frames, for example, and a percentage of them is getting damaged, that probably represents a more serious problem than the same percentage of 1500-byte frames getting damaged. So, it's better to use a total number of bytes rather than a total number of frames in the calculation.) When troubleshooting at the Data Link Layer, which deals with frames rather than bits, you can't actually determine a bit error rate, but you can at least get a rough estimate by considering the number of CRC errors compared to the number of megabytes received. Some Cisco documentation simply states that a problem exists if input errors are in excess of 1 percent of total interface traffic. This is easier to remember, but it's actually just as hard to comprehend. The documents don't specify whether you should compare the input errors to the number of frames or the number of bytes received. If they means frames, then we have the problem already mentioned (no accounting for variable frame sizes). If they mean bytes, then 1 percent is very high. On a loaded network, 1 percent of total bytes represents a very high bit-error rate. You may want to use a number less than 1 percent. When troubleshooting input errors, you should also consider a timeframe and whether there's been a burst of errors and how long the burst has lasted. The telco practice is to report total errors along with errored seconds, for example. _______________________________ Priscilla Oppenheimer www.troubleshootingnetworks.com www.priscilla.com Lupi, Guy wrote: > > I remember looking at a link on Cisco's web site that stated an > acceptable > threshold for CRC errors on an interface. I believe it was > something like > CRCs could not exceed .001% of the total input packets on the > interface. > Has anyone else seen this link, or one like it? I am trying to > determine > the threshold for an alarm notification when polling for > iferrors. > > Guy H. Lupi > > Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=59493&t=59477 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
