When the Fibre Channel v1.0 Core is running at 2 Gb/s, it does not always transmit at the minimum IFG on transmit side. Specifically, when an R_RDY is received during the IFG, the Credit is incorrectly checked twice, which causes the IFG to increase by 2/3 cycles. The problem occurs only if there are R_RDY's during the IFG.
The core is not affected if it is configured to run at 1 Gb/s or Multispeed at 1 Gb/s.
To resolve this issue, install the patch that is available in the LogiCORE Fibre Channel v1.0 Core Release Notes and Known Issues Answer Record (Xilinx Answer 19527).
After you have installed the patch, the Fibre Channel v1.0 Core will correctly transmit at the minimum IFG when operating at 2 Gb/s.