We propose a novel model of impairment aware routing in multi bit rate elastic optical networks. The routing constraint with respect to optical signal to noise ratio (OSNR) is established by introducing the concept of the equivalent noise figure (ENF) for optical links between network nodes. The ENF, which is equal to the OSNR increment between nodes, is calculated based on modulation format and bit rate assigned to lightpath. The look up table is initially populated by calculated values until it is eventually updated by measurement results. This concept is applicable in dynamic optical networks where bit rate and modulation format of any lightpath can be dynamically adjusted and can be employed by using extended open flow concept.