code: def _notify_subnet_create(resource, event, trigger, **kwargs):
      
      context = kwargs['context']
      subnet = kwargs['subnet']
      l3plugin = bc.get_plugin(L3_ROUTER_NAT)
      for router in l3plugin.get_routers(context):
          if (router['external_gateway_info'] and
                  (router['external_gateway_info']['network_id'] ==
                   subnet['network_id'])):
              router_data = {'router': router}
              l3plugin.update_router(context, router['id'], router_data)
docs: Called when a new subnet is created in the external network