When I target Virtex-6 FPGA in the 10-Gigabit Ethernet MAC version 9.3 and earlier, block RAM instances do not comply with all asynchronous clocking conflict avoidance requirements as described in the Virtex-6 FPGA Memory Resources User Guide (UG363):
http://www.xilinx.com/support/documentation/user_guides/ug363.pdf.
This problem only exists when the example design local link FIFO is used and could result in memory collisions and erroneous behavior.
This issue has been corrected in the 10-Gigabit Ethernet MAC version 9.3 rev1 and later available starting in ISE design tools 11.5.