33.1 Overview of Hierarchical Routing

Hierarchical routing was mainly devised, among other things, to reduce memory requirements of simulations over very large topologies. A topology is broken down into several layers of hierarchy, thus downsizing the routing table. The table size is reduced from $n^{2}$, for flat routing, to about log n for hierarchical routing. However some overhead costs results as number of hierarchy levels are increased. Optimum results were found for 3 levels of hierarchy and the current ns implementation supports upto a maximum of 3 levels of hierarchical routing.

To be able to use hierarchical routing for the simulations, we need to define the hierarchy of the topology as well as provide the nodes with hierarchical addressing. In flat routing, every node knows about every other node in the topology, thus resulting in routing table size to the order of $n^{2}$. For hierarchical routing, each node knows only about those nodes in its level. For all other destinations outside its level it forwards the packets to the border router of its level. Thus the routing table size gets downsized to the order of about log n.