A network interface consists of two major components, the media-independent MAC component and the media-specific PHY component. The Intel i217-LM is a gigabit Ethernet PHY component. It is used with the MAC component that is built into Intel's various chipsets. The i210, on the other hand, is a standalone, integrated MAC+PHY, gigabit Ethernet solution.
From a performance standpoint, both of these solutions should offer roughly the same throughput. To be honest, I typically just plug the cable into the left-most connector and go; I rarely look to see which is which and I have never noticed any performance differences.
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