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Jumbo Frames and MTU size


Hi, I have an Intel PRO/1000 MT connected directly to a device that can send IP packets with different length (and with the "don't fragment" bit set). I noticed a considerable difference between the MTU size reported by the adapter and actual maximum packet size that can be received dependent on the Jumbo Frames parameter. When I deactivate Jumbo Frames, the MTU size is 1500 and the max. packet size is 1504, which sounds OK for me. But when I set the Jumbo Frames to 9014, the reported MTU size is 9000, but the adapter is capable of receiving up to 9974 bytes. Can anyone tell me where that difference comes from?

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I can't explain the entire difference in sizes, but I can give some examples of why the size you configure on the NIC is often different than what you configure on your switch. For example, the size configured for the NIC does not include 4 bytes for CRC, nor the 4 bytes added on for VLAN or Quality of Service tagging. The Ethernet adapter still handles packets with those extra bytes in there, but the setting only corresponds to the Ethernet frame with those extra bytes not counted. I guess you could say the discrepancy boils down to a different way of counting.

In any case, the maximum frame size on the switch needs to be set at least 4 bytes higher than the adapter setting to handle CRC plus the setting needs to be set even higher to handle other protocols that increase the size of the Ethernet frame. For example, if the adapter is configured for 9014 bytes, the switch setting will have to be set for at least 9018 bytes. The switch will have to be configured for at least 9022 bytes if you have VLAN tags.

I hope this helps.

Mark H