'As no floatingip will be set on the rfp device. Get floatingip from
the route of fip namespace.'

def get_router_cidrs(self, device):

    if (not self.fip_ns):
        return set()
    fip_ns_name = self.fip_ns.get_name()
    fip_2_rtr_name = self.fip_ns.get_int_device_name(self.router_id)
    device = ip_lib.IPDevice(fip_2_rtr_name, namespace=fip_ns_name)
    if (not device.exists()):
        return set()
    if (self.rtr_fip_subnet is None):
        self.rtr_fip_subnet = self.fip_ns.local_subnets.allocate(self.router_id)
    (rtr_2_fip, _fip_2_rtr) = self.rtr_fip_subnet.get_pair()
    exist_routes = device.route.list_routes(lib_constants.IP_VERSION_4, via=str(rtr_2_fip.ip))
    return {common_utils.ip_to_cidr(route['cidr']) for route in exist_routes}
