A network unit for use in a distributed bridging fabric has a multiplicity of user ports for the transmission of data frames to and from an external network and at least one fabric port for the transmission of frames between the network unit and another unit in the fabric. The network unit has a forwarding database for containing entries each including a media access control address, and a lookup engine organized for the insertion of an entry into the forwarding database when the network unit receives a data packet at a user port. The unit broadcasts via each fabric port of an "address added" message identifying a respective media access control address. The lookup engine also responds to such an "address added" message received from another unit to make a corresponding entry in the database and to annotate an entry to indicate activity of an address in response to an address which is already in the database. The network unit has an aging engine organized for the polling of entries in the database. The aging engine refreshes an entry for which the respective media access control address is annotated as active and is organized for the selective removal of inactive entries from the database.