A general best practice is to have a solid return path beneath all MDI signal traces. This return path can be a continuous ground or DC power plane. Reducing the width of the return path can potentially affect the impedance of the signal trace. This effect is more prominent when the width of the return path is comparable to the width of the signal trace. Avoid breaks in return path between the signal traces at all cost. A signal crossing a split plane may cause unpredictable return path currents and could impact signal quality and result in emissions issues.

Figure 12-2 Differential Signal Pair and Plane Crossing