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 atiL2BrTpFdbAddress has been seen. A value of '0' indicates that the port number has not been learned but that the bridge does have some forwarding/filtering information about this address . Implementors are encouraged to assign the port value to this object whenever it is learned even for addresses for which the corresponding value of atiL2BrTpFdbStatus is not learned(3).