In some situations, even the most judicious use of EIGRP scalability tools cannot improve the way a network behaves. Examples of these scenarios (some of them covered in the initial case studies) include the following:
Logically structured networks with no IP addressing structure or hierarchy
Large networks with no logical structure (for example, no core/distribution/access layer)
Networks with an extremely large number of routes (usually as a result of scenario 1)
In all these scenarios, you cannot design a scalable network using only EIGRP. You have to combine the benefits of EIGRP with benefits of other routing protocols to make the network stable and scalable.
EIGRP needs to ...