The following applies If you are using 10GBASE-KR with Auto-Negotiation enabled in the 10-Gigabit Ethernet PCS/PMA or AXI 10G Ethernet core, and targeting UltraScale GTH.
There is an update to the UltraScale GTH attributes required to improve Auto-Negotiation performance and reliability across a backplane.
The GTHE3 RXCDR_CFG2 bit 9 needs to change from a 1 to a 0.
In Vivado 2014.4.1 an XDC constraint can be added to the user XDC file make this change:
Without it set, Auto-Negotiation might require numerous restarts to complete and could therefore take several seconds.
In Vivado 2015.1, this constraint has been added to the core level XDC so that no user updates are required.