An Efficient Algorithm for OSPF Subnet Aggregation

Multiple addresses within an OSPF area can be aggregated and advertised together to other areas. This process is known as address aggregation and is used to reduce router computational overheads and memory requirements and to reduce the network bandwidth consumed by OSPF messages. The downside of address aggregation is that it leads to information loss and consequently sub-optimal (non-shortest path) routing of data packets. The resulting difference (path selection error) between the length of the actual forwarding path and the shortest path varies between different sources and destinations. This paper proves that the path selection error from any source to any destination can be bounded using only parameters describing the destination area. Based on this, the paper presents an efficient algorithm that generates the minimum number of aggregates subject to a maximum allowed path selection error. A major operational benefit of our algorithm is that network administrators can select aggregates for an area based solely on the topology of the area without worrying about remaining areas of the OSPF network. The other benefit is that the algorithm enables trade-offs between the number of aggregates and the bound on the path selection error. The paper also evaluates the algorithm’s performance on random topologies. Our results show that in some cases, the algorithm is capable of reducing the number of aggregates by as much as 50% with only a relatively small introduction of maximum path selection error.