Tiered:http://www.angelfire.com/mech/phattony/wan_topologies.htm
Tiered WAN topologies are similar to the hierarchical hybrid topologies used with LANs. In a tiered WAN topology, WAN sites connected in a star or ring formations are interconnected at different levels, with the interconnection points being organized into layers.
Variations on this topology abound. Indeed, flexibility makes the tiered approach quite practical. A network architect can determine the best placement of top-level routers based on traffic patterns or critical data paths. In addition, tiered systems allow for easy expansion and inclusion of redundant links to support growth. On the other hand, their enormous flexibility means that creation of tiered WANs requires careful consideration of geography, usage patterns, and growth potential.