How to delete stale routers?

Issue

• Unable to delete stale router whose ports are down.

• Removal of the internal port associated with router fails with below error.

  (overcloud) [stack@director ~]$ openstack router remove port <router_id> <port_id>
  HttpException: 500: Server Error for url: https://domain.com:13696/v2.0/routers/<router_id>/remove_router_interface, Request Failed: internal server error while processing your request.