4.4. Responsibility for and configuration of aggregation

The domain which has been allocated a range of addresses has the sole authority for aggregation of its address space. In the usual case, the AS will install manual configuration commands in its border routers to aggregate some portion of its address space. An domain can also delegate aggregation authority to another domain. In this case, aggregation is done in the other domain by one of its border routers.

When an inter-domain border router performs route aggregation, it needs to know the range of the block of IP addresses to be aggregated. The basic principle is that it should aggregate as much as possible but not to aggregate those routes which cannot be treated as part of a single unit due to multi-homing, policy, or other constraints.

One mechanism is to do aggregation solely based on dynamically learned routing information. This has the danger of not specifying a precise enough range since when a route is not present, it is not always possible to distinguish whether it is temporarily unreachable or that it does not belong in the aggregate. Purely dynamic routing also does not allow the flexibility of defining what to aggregate within a range. The other mechanism is to do all aggregation based on ranges of blocks of IP addresses preconfigured in the router. It is recommended that preconfiguration be used, since it more flexible and allows precise specification of the range of destinations to aggregate.

Preconfiguration does require some manually-maintained configuration information, but not excessively more so than what router administrators already maintain today. As an addition to the amount of information that must be typed in and maintained by a human, preconfiguration is just a line or two defining the range of the block of IP addresses to aggregate. In terms of gathering the information, if the advertising router is doing the aggregation, its administrator knows the information because the aggregation ranges are assigned to its domain. If the receiving domain has been granted the authority to and task of performing aggregation, the information would be known as part of the agreement to delegate aggregation. Given that it is common practice that a network administrator learns from its neighbor which routes it should be willing to accept, preconfiguration of aggregation information does not introduce additional administrative overhead.

Implementation note: aggregates which encompass the class D address space (multicast addresses) are currently not well understood. At present, it appears that the optimal strategy is to consider aggregates to never encompass class D space, even if they do so numerically.