Figure 14 The T1 Nsfnet Backbone 1990 Regionals Added after 1990 Are Shaded

Supporting the NSFNET backbone routers was an evolutionary exercise, requiring close collaboration between the network operators and the code developers. This proved to be one of the great strengths of the team from Merit, IBM, and MCI, as they provided ongoing engineering to support a network that grew by as much as 500 percent per year.

The rapid growth of the NSFNET, coupled with a requirement that the regional networks directly connect to each other rather than relying on the NSFNET backbone, led the NSFNET operators to introduce a rudimentary policy-routing system.

Among other points, the policy-routing system involved filtering all networks advertised to the NSFNET from the regionals based on both network prefix (at this point, it was actually network number because Internet routing was still classful) and autonomous system number. The policy-routing system also set the metric of all accepted routes. These functions were performed in consultation with a distributed policy routing database (PRDB).

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