When using the LogiCORE Ethernet 1000BASE-X PCS/PMA or SGMII v10.2 Core Spartan-6 example design, some implementations fail with a MAP Place error similar to the following:
ERROR:Place:392 - Can not place component gmii_data_bus1.delay_gmii_txd1
The UCF for the example is locking down the GMII bus to an I/O Bank and leaving the tools to auto-place. In the case of the failure, the tools are auto-placing illegally.
In a customer design, the I/Os should all be LOC-ed to legal locations as per the PCB layout, and this will not be an issue.