The block RAM is not being initialized correctly in Spartan-6 hardware. This resulted in the transmitted PTP frames being corrupted. This only effects Spartan-6 designs and does not effect other target devices.
This issue has been resolved in the version 2.2 Rev 1 patch. For the patch and download instructions, see:
(Xilinx Answer 33305) LogiCORE IP Ethernet AVB Endpoint v2.2 - Release Notes and Known Issues for ISE 11.3 software