'Return the provider driver handle for a router id.'

def get_provider_for_router(self, context, router_id):

    driver_name = self._stm.get_provider_names_by_resource_ids(context, [router_id]).get(router_id)
    if (not driver_name):
        router = self.l3_plugin.get_router(context, router_id)
        driver = self._attrs_to_driver(router)
        driver_name = driver.name
        self._stm.add_resource_association(context, plugin_constants.L3, driver_name, router_id)
    return self.drivers[driver_name]
