USRP2 project on ISE11.1 version

Dear all,

          I was trying to run the usrp2 project on ISE v 11.1. My

project compilation fails with errors as follows,
Error: MapLib : 30 - LOC constraint V15 on PHY_CLK is invalid : no such
on the device. to bypass this error set the environment variable
there are several (301 Maplib errors and all suggest the same remedy).

Any body knows how to tackle the compilation problem of USRP2 project in
ISE11.1 version??

With Best Regards and Thanks in advance,