Align and FCS error

Unanswered Question
Mar 10th, 2008

We have 6509 CatOS switches. When we do " show port counters," that shows some Align and FCS errores. We checked the cable and speed/duplex, it looks good. How can we clear out/fix those errors.

I have this problem too.
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lamav Mon, 03/10/2008 - 11:18


Alignment errors are a count of the number of frames received that don't end with an even number of octets and have a bad CRC.

FCS error count is the number of frames that were transmitted/received with a bad checksum (CRC value) in the Ethernet frame. These frames are dropped and not propagated onto other ports.

When operating at half-duplex setting, some data link errors such as Frame Check Sequence (FCS), alignment, runts, and collisions are normal. Generally, a one percent ratio of errors to total traffic is acceptable for half-duplex connections. If the ratio of errors to input packets is greater than two or three percent, performance degradation may be noticed.

When operating at full-duplex, FCS, Cyclic Redundancy Checks (CRC), alignment errors, and runt counters should be minimal. If the link is operating at full-duplex, the collision counter is not active. If the FCS, CRC, alignment, or runt counters are incrementing, check for a duplex mismatch. Duplex mismatch is a situation where the switch is operating at full-duplex and the connected device is operating at half-duplex, or vice versa. The result of a duplex mismatch will be extremely slow performance, intermittent connectivity, and loss of connection. Take note that using "autonegotiation" on both ends may cause a problem between different vendor equipment due to the proprietary additions to the protocol that some vendors implement. Try hard-coding both ends for speed and duplex settings.

Other possible causes of data link errors at full-duplex are bad cables, faulty switch port, or NIC software/hardware issues.


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Istvan_Rabai Mon, 03/10/2008 - 11:40

Hi Ken,

Visctor's explanation is very good.

If you have checked speed and duplex settings and you still have problems, then check if you have at least CAT5 cable along the path and it does not exceed 100 m, including the patch cables.

Please also check if there are any high current cables in the proximity (within 1 m) of the LAN cable. Magnetic interference can cause unexpected errors as well.




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