If autonegotiation is supported and properly configured but a connection still cannot be established, turn off autonegotiation by setting the speed and duplex manually. Issue the set port speed and set port duplex commands. Be sure that both ends of the link are configured with the same speed and duplex settings.
Symptoms of a failed link include Frame Check Sequence (FCS) and alignment errors. The FCS error count is the number of frames that were transmitted/received with a bad checksum (Cyclic Redundancy Check (CRC) value) in the Ethernet frame. These frames are dropped and not propagated onto other ports. Alignment errors are a count of the number of frames received that do not end with an even number of octets and have a bad CRC.
Issue the show port command to see if these counters are incrementing on a CatOS switch.
Issue the show interface command to check for CRC errors on a Cisco IOS -based switch.
This is an example of the counters from the show port command from a CatOS switch: