check the speed/ duplex settings of the 6500 and the 3000. i have seen, even if set for auto, that the port will error out. if you have them both for auto, try to hard set them both, or visa versa. remember to re-enable the port first. could also be a bad port. try another port as well.
just several suggestions. maybe still something else, but these are a couple to start with.
Please verify the following:
1. Port where Concentrator is connected is configured for "Spantree portfast" enable? If so, disable it.
2. Cat 5 Cable issue - See if cable is good or not.
3. Make sure that the UDLD (Uni Directional Link Detection) is not set for the port.
NYC Department of Correction