The Virtex-4 and Virtex-5 Embedded Tri-mode Ethernet MAC supports half-duplex 10/100 Mbps MII and RGMII. When sending back to back frames in these modes you will not be able to get max throughput. In 10/100 MII/GMII mode, the min IFG will be 18 bytes. In 10/100 Mbps RGMII mode, the min IFG will be 26 bytes.
This only effects 10/100 Mbps half-duplex operation. The maximum throughput with a minimum IFG of 12 can be achieved in full duplex in the Embedded Tri-mode Ethernet MAC. At 1 Gbps, only full-duplex operation is supported. If needed, full thoughput can be achieved by using the soft Core Generator LogiCORE Tri-mode Ethernet MAC with the clock enable option.
Be aware that the LogiCORE Tri-mode Ethernet MAC core has the same reduced throughput behavior as the Embedded Tri-mode Ethernet MAC if it is used without the clock enable option.