In 13.2, the Kintex-7 GTX IBERT core generate errors in BitGen if you do not use an external clock source for the IBERT system clock. You see an error similar to:
ERROR:PhysDesignRules:798 - The network <q2_clk0_mgtrefclk> has an antenna.
Routing is incomplete.
ERROR:PhysDesignRules:9 - The network <q2_clk0_mgtrefclk> is only partially
routed.