According to the programming guide for the BCM57XX chipset (pg 133/pdf pg 198):
The BCM5705, BCM5788, BCM5721, BCM5751, and BCM5752 devices do not support Jumbo Frames and hence both
This chipset is used on a lot of dell laptops. The one that I use for work is Dell M65. The Dell D820 has the same problem.