This article is complementary to (Xilinx Answer 64835) which is for 1G/2.5G Ethernet PCS/PMA or SGMII for 7 Series devices.
For UltraScale and UltraScale+ devices, (Xilinx Answer 64835) is not sufficient and the AN restart can happen because the Transceiver initialization time is longer than for 7 Series devices.
Replace the RX elastic buffer file with the file of the same name in the attached files. The fix will be included in Vivado 2016.3.
Please note, the core is not affected if it is configured for 1000BASE-X only, or in SGMII mode without the fabric elastic buffer.