That was certainly a good explanation that helped me realize much better. After going through the link you referenced I feel I strongly understand the advantages of hierarchical FIB, therefore I asked why routers do not have such FIB by default?

It actually isn't a question of homework. I am just trying to understand the FIB design and design constraints / considerations, not much is said about FIB's, I have read most fib are flat and that having some kind of hierarchy can significantly improve performance and reduce table size. So, I would like to be referenced to an article or a debate arguing and speaking of FIB's design and architectures, as well maybe something that can answer both of my questions in the original post.