3.1 Hierarchical Routing

Hierarchical routing is a routing method based on hierarchical addressing.

Telephone networks have been around for over a century now. Earlier point-to-point direct links (trunkgroups) were set up to connect calls between different places; there was no routing involved. The need for routing arose for two primary reasons:

	Point-to-point links lead to the N2 problem, i.e., if there are N nodes in a network, we need N(N −1)/2 directly connected links; thus, as more and more cities (with multiple switches) offer telephone services, this problem grows significantly,
	It was recognized that some trunkgroups were less utilized compared to others; thus, if there were any way to take advantage of this by routing calls through less utilized trunkgroups, capacity expansion could be avoided.

Capacity expansion used to be very costly and still is in many cases. There is another impetus to arriving at some form of routing:

As the switching technology started to move from old mechanical switches to electromechanical switches, the possibility of switching being capitalized to perform some form of routing became more than a thought.