Can Longest Prefix Matching Make The Path Length Shorter?

It has been recognized that the existing routing architecture of today’s Internet is facing scalability problems. Although deployment of CIDR (Classless Inter-Domain Routing) slows the growth of global routing tables, LPM (longest prefix matching), which is greedily pursuing for more-specific prefixes, seriously restricts the advance of scalable routing. The prevalent LPM algorithm for routing lookup is considered the best path to the destination, which logically makes sense. However, we find that LPM algorithm does not necessarily provide shortest path length for routing lookup in most cases with comparison experiment. To find more general prefixes, we first present SPM (shortest prefix matching) algorithm, and then make a comparative evaluation between SPM and LPM by using the metric of average AS path length. In conclusion, we analyze the reasons that cause longer average AS path length for LPM algorithm in most cases and discuss the impact of LPM to scaling routing.