Either the value '0', or the port number of the port on which a frame having a source address equal to the value of the corresponding instance of ieee8021QBridgeTpFdbAddress has been seen. A value of '0' indicates that the port number has not been learned but that the device does have some forwarding/filtering information about this address (e.g., in the ieee8021QBridgeStaticUnicastTable). Implementors are encouraged to assign the port value to this object whenever it is learned, even for addresses for which the corresponding value of ieee8021QBridgeTpFdbStatus is not learned(3).