In ISE 12.2 software and later with the Tri-Mode Ethernet MAC v4.4 rev1, core timing is now met using the DCM to adjust the RGMII receive clock and data relationship.
In ISE 12.2 software and later with the Tri-Mode Ethernet MAC v4.4 rev2, core timing is met using a BUFG on the RGMII RX clock and IDELAYs and the RGMII RX data.
For more information, see
(Xilinx Answer 35279) LogiCORE IP Tri-Mode Ethernet MAC v4.4, v4.4rev1 and v4.4rev2 - Release Notes and Known Issues for ISE 12.1 and ISE 12.2 software