TCAM and CAM memory usage inside networking devices
As this is networking blog I will focus mostly on the usage of CAM and TCAM memory in routers and switches. I will explain TCAM role in router prefix lookup process and switch mac address table lookup.
However, when we talk about this specific topic, most of you will ask: how is this memory made from architectural aspect?
How is it made in order to have the capability of making lookups faster than any other hardware or software solution? That is the reason for the second part of the article where I will try to explain in short how are the most usual TCAM memory build to have the capabilities they have.
CAM and TCAM memory
When using TCAM – Ternary Content Addressable Memory inside routers it’s used for faster address lookup that enables fast routing.
In switches CAM – Content Addressable Memory is used for building and lookup of mac address table that enables L2 forwarding decisions. By implementing router prefix lookup in TCAM, we are moving process of Forwarding Information Base lookup from software to hardware.
When we implement TCAM we enable the address search process not to depend on the number of prefix entries because TCAM main characteristic is that it is able to search all its entries in parallel. It means that no matter how many address prefixes are stored in TCAM, router will find the longest prefix match in one iteration. It’s magic, right?
Image 1 shows how FIB lookup functions and points to an entry in the adjacency table. Search process goes through all entries in TCAM table in one iteration.
Router
In routers, like High-End Cisco ones, TCAM is used to enable CEF – Cisco Express Forwarding in hardware. CEF is building FIB table from RIB table (Routing table) and Adjacency table from ARP table for building pre-prepared L2 headers for every next-hop neighbour.