Gateway devices at different sites of a primary Anycast network provide access to the sites by advertising a first set of Anycast addresses. A secondary shadow Anycast network advertises different second sets of Anycast addresses from the different sites in order to predetermine traffic shifts that occur as a result of changing one or more of the second set of Anycast addresses that are advertised from one or more of the sites. A traffic shifting device may implement a predetermined traffic shift in the primary Anycast network by selecting a particular second set of network addresses that produces a traffic shift at least equal to the predetermined traffic shift, mapping the particular second set of network addresses to a modified first set of addresses, and modifying routing in the primary Anycast network by advertising the modified first set of addresses instead of the first set of addresses.