Excessive CRC/Alignment Errors on port xx

Excessive CRC… means a bad negotiation.
Is port speed/duplex correct ?
Disable lacp on port that do not need it.

First of all you should to check the cable connected to the port. Do not trust multimeters – use normal network analizer to measure NEXT, attenuations, wrong pairing etc.
Next, check the speed/duplex settings on both sides.
Next, monitor network traffic of your host, connected to the port. You can use WireShark (freeware tools, Google for it) for this purpose (it is available for *NIX and Windows as well).
Next localize the application generated this traffic. Tools will depend on OS you are using.

How many errors are too many?

Data link errors such as CRC errors, alignment errors, and runts will occur on healthy networks. How do you distinguish between a reasonable number of these errors and too many? A rule of thumb is one error in 5,000. For example, on average, for every 5,000 packets received you should have no more than one receive error (CRC, alignment, runt, short, giant, or too long). And on average, for every 5,000 packets transmitted, you should have no more than one transmit error (late collision, excessive collision, late event, excessive deferral, or loss of carrier). At higher rates of errors, users will probably perceive the network’s performance as being poor.

One data link error in 5,000 does not necessarily indicate a perfectly-performing network. Rather, it indicates a network where the errors are probably not causing serious performance problems that are apparent to the users.


Most common cause for alignment errors is either speed mismatch or duplex mismatch. If the switch port is hard set to 100 full duplex, for example, and the device hooked into it is set to auto or any other setting besides 100 full duplex you will get alignment errors. You should have both devices set to auto or set to the same speed/duplex combo.

I’d start with checking that unless swaping cables is very easy.

Any communications errrors will slow down the speed of the link. As the PC/Switch will have to resend the information that didn’t make it. The more errors, the slower it gets.

The usual cause of this is either:

1) Buggy NIC drivers.
2) A physical cable fault.
3) Mismatched speed/duplex settings.
4) Sending tagged VLAN packets to a port that’s not expecting them.

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