'get exist route distinguisher'

def get_exist_rd(self):

    if (not self.vrf_af_info):
        return None
    for vrf_af_ele in self.vrf_af_info['vpnInstAF']:
        if (vrf_af_ele['afType'] == self.vrf_aftype):
            if (vrf_af_ele['vrfRD'] is None):
                return None
            else:
                return vrf_af_ele['vrfRD']
        else:
            continue
    return None
