Advantages of hierarchy system on Routing Protocols The OSPF has a hierarchy system that separates a single Autonomous System into individual areas, while EIGRP has not a hierarchy system, it prefer to handle with an entire Autonomous System, right? What are advantages of using a Hierarchy System? What if there, Why EIGRP has not a Hierarchy System?

This has to do with the algorithms used. OSPF and IS-IS use Dijkstra's algorithm which requires a hierarchy. EIGRP uses a proprietary algorithm (DUAL) which does not. The DUAL algorithm is diffusing (the "D" in DUAL) so that it doesn't require a hierarchy.

EIGRP can be set up in a sort of hierarchy with stub areas, but it doesn't fundamentally need a hierarchy the way OSPF or IS-IS do.

With Dijkstra's algorithm, each router has a complete understanding of an area, but DUAL only knows about the next hop for a route.