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Packets -- if sent over MTU 9000 (jumbo frames) -- will automatically get repacketed to fit the lowest MTU they are sent through. If possible, using isolated networks for Jumbo vs. standard frames is the best practice, but not always possible. If you have a network with VLANs on it and it's set to 9000, but some of the VLANs have attached devices that only handle 1500, they will get reframed. The overhead seems really small, as far as I can tell, though I've not measured it in any quantitative way. For extensive discussions on this, google "MTU packing efficiency".