Auxiliary Power Option A (Front Aux or FAUX Pin Method)
The behavior and the requirements depend on the following cases.
VAUX within PoE Range
In the following discussion, pay close attention to Fig. 14.2. In general, this method is said to support only VAUX between 42 and 57 V. But there are ways around that restriction as discussed here.
FIGURE 14.2 Front Aux method in more detail.
It is clear that if VPSE is present (i.e., between 44 and 57 V), VAUX needs to be greater than VPSE for the OR-ing diode D1 in Fig. 14.2 to conduct. Therefore ...