Although the jumbo MTU has been configured for class-default, you cannot pass a frame size larger than 2300 bytes through the Nexus 5000 switch and the Nexus 2000 FEX.
The CoS value may conflict with the existing MTU value.
CoS 7 is used internally for controlling traffic between the Nexus 5000 switch and the Nexus 2000 FEX. The MTU value for the traffic with CoS 7 is set to a fixed value. You must check that the incoming traffic is marked with CoS 7.
Use any CoS value other than 7 to avoid this limitation.