When interfacing Xilinx source core with the IXP2800 device, and if you have Xilinx Source core configured to send periodic training patterns, the IXP2800 device may indicate DIP4 errors over the length of the operation.
It has been reported that IXP2800 has a problem receiving periodic training patterns. This is an issue that has to be addressed by the Intel IXP2800 developer. For proper configuation of the IXP2800, please refer to the appropriate documentation provided by Intel.
The immediate workaround is to disable sending of periodic training patterns on the Xilinx source core. Note that setting the static configuration signal AlphaData to zero configures the core to not periodically send training patterns.