This selects which route types are still advertised while the device is in Overload state: null(0, the default) specifies no exception, external(1) still advertises imported external routes, interlevel(2) still advertises inter-level leaked routes, and externalandinterlevel(3) advertises both. Admins use this to keep essential redistributed or leaked routes reachable even while the router is deliberately avoided for general transit.