The example design ucf for the LogiCORE Tri-Mode Ethernet MAC v3.3 and v3.4 is missing a ucf period constraint for the rx clock.
The example design ucf contains the following two lines:
NET "*rx_gmii_mii_clk_int" TNM_NET = "clk_rx_int";
TIMEGRP "rx_clock" = "clk_rx_int";
But, the ucf is missing the period constraint. The following line should be added to the ucf:
TIMESPEC "TS_rx_clock" = PERIOD "rx_clock" 8000 ps HIGH 50 %;
This issue will be been fixed in the next release of the LogiCORE Tri-Mode Ethernet MAC v3.5, scheduled to be released in ISE 10.1.