1. A method, comprising: maintaining a first client list identifying a first set of client devices associated with a first content distribution network; obtaining a second client list identifying a second set of client devices associated with a second content distribution network; producing, from elements of the first client list and the second client list, a preferred client list identifying preferred client devices for which the second content distribution network processes client requests; routing a client request associated with a client device to one of, the first content distribution network and the second content distribution network based, at least in part, on the preferred client list; and periodically refreshing the preferred client list by selectively obtaining the second client list and filtering the first client list and the second client list to reproduce the preferred client list.