You must modify the default example design generated by the CORE Generator in the ISE Design Suite 13.2 to make sure that the GTRXRESET pin is asserted until the PLL has locked as follows:
In the *_block.vhd file:
Replace:
mgt_rx_reset <= cbm_rx_reset or reset156;
With:
mgt_rx_reset <= cbm_rx_reset or reset156 or (not mgt_plllocked);
In the *_block.v file:
Replace:
assign mgt_rx_reset = cbm_rx_reset || reset156;
With:
assign mgt_rx_reset = cbm_rx_reset || reset156 || ~mgt_plllocked;