1. A method comprising: aggregating, with an application-layer traffic optimization (ALTO) server that stores network topology information for a network of one or more endpoints, the endpoints into a set of one or more provider-defined identifiers (PIDs), wherein each PID represents a subset of the endpoints, and wherein each endpoint associated with a particular PID in the set of PIDs is a member of the same type; assigning a PID-type attribute to each of the PIDs, wherein the assigned PID-type attribute specifies whether the subset of endpoints associated with the PID are consumers of a content delivery service or are providers of a content delivery service; generating, with the ALTO server, an ALTO network map that includes respective PID entries to describe the PIDs, wherein each PID entry includes a PID-type field that stores the assigned PID-type attribute for the PID described by the PID entry; receiving a request for an ALTO cost map for the network from an ALTO client with the ALTO server, wherein an ALTO cost map includes one or more cost entries that each specify a different pair of the PIDs and an associated cost value that represents a cost to traverse a network path between the members of the PID pair; setting cost values for cost entries that specify pairs of PIDs that include members assigned the same PID-type attributes to prevent selection, for content delivery to host devices associated with a first PID of the PIDs, of a second PID of the PIDs assigned the same PID-type attribute as the first PID; and outputting, with the ALTO server to the ALTO client in response to the request for the ALTO cost map, the cost entries as an ALTO cost map for the network.