Information reduction may imply a reduction in granularity of policy control - after information is collapsed, the same policies will apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information that it will place in the Adj-RIBs-Out by any of the following methods:
Destination IP addresses can be represented as IP address prefixes. In cases where there is a correspondence between the address structure and the systems under control of an autonomous system administrator, it will be possible to reduce the size of the NLRI carried in the UPDATE messages.
AS path information can be represented as ordered AS_SEQUENCEs or unordered AS_SETs. AS_SETs are used in the route aggregation algorithm described in 9.2.4.2. They reduce the size of the AS_PATH information by listing each AS number only once, regardless of how many times it may have appeared in multiple AS_PATHs that were aggregated.
An AS_SET implies that the destinations listed in the NLRI can be reached through paths that traverse at least some of the constituent autonomous systems. AS_SETs provide sufficient information to avoid routing information looping; however their use may prune potentially feasible paths, since such paths are no longer listed individually as in the form of AS_SEQUENCEs. In practice this is not likely to be a problem, since once an IP packet arrives at the edge of a group of autonomous systems, the BGP speaker at that point is likely to have more detailed path information and can distinguish individual paths to destinations.