While this question is very broad I have managed to provide a couple of links below and a quick read would prove handy.
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Vignesh R P
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Well, for starters, any features being used on the 2821 that are not found on the 3850. Also, how large a route table is being used by the 2821. (The former should be able to deal with full Internet route tables, but the 3850 might be unable to do so.)