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 vlanTpFdbAddress 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 vlanStaticUnicastTable). Implementors are encouraged to assign the port value to this object whenever it is learned even for addresses for which the corresponding value of vlanTpFdbStatus is not learned(3).