It's important that both link lights, one at each end, be lit. That indicates that there is a correctly wired signal path between both devices. On the other hand, the presence of the link lights merely means that the segment is correctly wired. Since the link test pulse operates more slowly than actual Ethernet signals, the link lights are not always a guarantee that Ethernet signals will work over the segment. Odds are good that a correctly wired segment will work, but if the signal crosstalk on the segment is too high, then it may not work despite the presence of the link lights.