A Dynamic Longest Prefix Matching Content Addressable Memory for IP Routing

An internet protocol (IP) router determines the next hop for a packet by finding the longest prefix match. This lookup often occurs in ternary content addressable memory (TCAM), which allows bit masking of the IP address. In this paper, an internet protocol content addressable memory (IPCAM) circuit that directly determines the longest prefix match to the stored address is described. The proposed IPCAM produces an encoded prefix match length that is limited by the prefix mask. Entries need not be sorted in order. One of the proposed IPCAM entries replaces on average 22 TCAM entries. Consequently the longest prefix matching CAM is less than 1/10 the size of the equivalent TCAM and dissipates 93.5% less dynamic power. The encoded outputs drive a priority encoder to determine the longest prefix match in the IPCAM arrays. A priority encoder circuit architecture appropriate to the unsorted IPCAM entries is also presented.