A fast ternary CAM design for IP networking applications

In this paper we describe a VLSI implementation and complete circuit design of a fast ternary CAM (TCAM). TCAMs are commonly used to perform routing lookups in the backbone of IP networks and small gateways. Our TCAM is designed to have a greater capacity and speed than any commercial offering at this time. In contrast with existing TCAM approaches, our TCAM allows complete flexibility in the location where any new entry is inserted. This is achieved by a novel longest prefix match (LPM) determination circuit, whose delay increases logarithmically with the number of bits to be looked up. We have implemented our TCAM with 512 bits of prefix entry with 512 bits of destination information, allowing it to implement large address lookups as well as quality of service mechanisms. This would make our TCAM design particularly suitable for IPv6 routing lookup applications. The speed improvement of our TCAM over currently available TCAMs results from various carefully selected VLSI architectural and implementation choices. The TCAM size is 21 Mb and is broken up into a regular grid of 13x13 smaller TCAM blocks for improved speed characteristics. Routing lookup operations use a heavily pipelined approach for maximum throughput, while ensuring a lookup latency of 3 clock cycles. Individual match lines in these blocks are split into 4 sections to reduce RC delay in the lookup process. Our LPM determination circuit is implemented using an efficient wired-NOR circuit for further reduced delay. Sense amplifiers are utilized in the LPM and SRAM sections of the TCAM and are located in the center of each TCAM subblock in order to improve lookup speed. We have implemented and validated our design using state-of-the-art circuit analysis and design tools. We have also generated mask layouts of the entire TCAM design using current layout tools. The complete TCAM circuit design is approximately 18mm on a side, with a total capacity of 21Mb. Our TCAM has an ability to perform routing lookups at a line rate of 76.8Gb/s which is twice as fast as the fastest commercially available TCAM today.