The following image shows two metal traces on a dielectric slab with a PEC ground plane. The remainder of the enclosure has been assigned an approximate open boundary condition.
The ends of the traces have the same port index, indicating common-mode excitation. Running the simulation and viewing the port field produces the following:
These fields show common-mode excitation: the field vectors are directed away from both traces, indicating that both traces are held at a positive voltage with respect to the ground plane. If you change the sign on the right-hand port and rerun the simulation, you obtain the following field annotation.
The field vectors are directed out of the positive trace (indicating positive voltage) and into the negative trace (indicating negative voltage) as expected for differential mode excitation. This example illustrates how using different voltage constraints on a single geometry results in different modes. In this case, the modes are not degenerate: the wave-number for the differential mode case is higher than that of the common mode case.