The GTH Transceiver default attribute settings are different between the ES and Production silicon versions of Virtex-6 HXT devices.
The ChipScope IBERT tool (released with ISE software versions 12.x/13.1 only) support the ES silicon settings of GTH Transceivers. The IBERT version (released with ISE 13.1 Update) supports the Production silicon as well as ES silicon settings of Virtex-6 GTH Transceivers.