The index (ethFwdIndex) in the ethFwdFuncTable of the
forwarding function that this interface belongs to. The
value -1 denotes that the interface does not belong to any
forwarding function. If the referenced forwarding function
is deleted, then the value of this object is automatically
reset to -1. Note that the interface can only belong to a
forwarding function on the same device as the interface. An
interface group and its member interfaces always refer to
the same forwarding function.