Another viable approach to generate hierarchical topologies is the bottom-up approach. The accuracy of this approach with respect to generating representative Internet topologies has yet to be validated. Nonetheless, we believe it is an alternative mechanism to generate hierarchical topologies. The interesting question to be answered with this approach is: how can we infer topological characteristics at the AS-level from known topological information at the router-level. BRITE provides a model that generates hierarchical topologies following this approach.
In this model, BRITE first generates a router-level topology using any of the available models (router Waxman, imported file, etc.). Once this topology has been constructed, BRITE assigns to each AS node (level-2 node) a number of routers according to an assignment type specified by the user. With this number of assigned routers to an AS node, BRITE groups that many nodes from the router topology following a grouping method specified also by the user as a parameter to BRITE. The next two subsections describe the assignment types and grouping mechanisms provided as a base bottom-up model by BRITE.