When implementing the Virtex-6 Embedded Tri-mode Ethernet MAC Wrapper in 11.2, the following error has occasionally been seen:
ERROR:Place:1153 - A clock IOB / BUFGCTRL clock component pair have been found that are not placed at an optimal clock IOB / BUFGCTRL site pair.
This error message results from the placer not finding an optional location. This error can be resolved by LOCing the clock pin in the error message to a specific legal location. This issue will be resolved in 11.3.