An Efficient Technique for Longest Prefix Matching in Network Routers

Abstract

Network Routers find most defined path for an arriving packet by the destination address in the packet using longest prefix matching (LPM) with Routing table entries. In this paper we propose a new Ternary Content Addressable Memory (TCAM) based system architecture for the LPM problem in routers. The proposed architecture eliminates sorting of table entries during table update [1][2]. It also eliminates the priority encoder needed to find the longest prefix match in conventional techniques .This has advantage in large capacity routing tables as proposed technique uses a priority encoder only of size equal to the number of bits in destination address to find the longest prefix length. To implement the proposed method for LPM, TCAM cell is modified by including two control transistors which control connection of cell either with Bit Match Line (BML) or with Word Match Line (WML). Functionality of modified cell is verified by simulating 32-bit TCAM word in UMC 180 nm technology in Spectre. Difference in search cycle time has been observed to be comparable to the conventional TCAM. The proposed technique completely reduces the LPM problem to only three search cycles in proposed TCAM memory architecture. As in recent times router table update rate has increased along with its capacity, proposed architecture is expected to be advantageous over conventional in large capacity and high update rate routing tables, due to elimination of sorting [2] and storage of any extra information on new entry [3].