Look-up table for a bridge in a data communications system

Abstract

A look-up table for use in a bridge employing negative filtering for interconnecting first and second networks in a data communications system. Associated with each data transmission is a source and destination address each having a total of (m+n) binary data bits, the bridge passing the data transmission from one network to the other unless the destination address is found in the look-up table and corresponds to the sending network. The look-up table comprises an encoding means responsive to an (m+n) bit address for producing first and second codes having respectively m and n bits of data which together allow the (m+n) bit address to be decoded unambiguously, a random access memory (RAM) coupled to the encoding means and having m addressable locations each for storing at least n bits of data, and a contents addressable memory (CAM) coupled to the encoding means and having a plurality of addressable locations each for storing at least (m+n) bits of data. If the location in RAM whose address is equal to the first code is available, then the second code is stored in the location. Otherwise, if the second code is not stored therein already, then all (m+n) bits of the (m+n) bit address are stored in an available location in the CAM unless they are stored therein already. A destination address is looked up by reading the contents of the CAM as well as the contents of the location in the RAM whose address is equal to the first code.