1. For a multi-user datacenter in which a plurality of user-managed logical routers are implemented, a method comprising: receiving configuration data connecting a user-managed logical router to a datacenter-managed logical router in order to connect the user-managed logical router to devices external to the datacenter, wherein the datacenter-managed logical router comprises (i) a distributed routing component implemented across a plurality of physical machines in the datacenter and (ii) one or more centralized routing components that are each implemented on a different physical machine, wherein each of the distributed and centralized routing components of the datacenter-managed logical router has a separate routing table; and for each route configured for the user-managed logical router: adding, to the routing table of a distributed routing component of the datacenter-managed logical router, a route for a same prefix as the route configured for the user-managed logical router and having a next hop address of an interface of the user-managed logical router; and adding, to the routing table of each centralized routing component of the datacenter-managed logical router, a route for the same prefix as the route configured for the user-managed logical router and having a next hop address of an interface of the distributed routing component of the datacenter-managed logical router.