which a frame having a source address equal to the value
of the corresponding instance of gDot1qTpFdbAddress 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 gDot1qStaticUnicastTable).
Implementors are encouraged to assign the port value to
this object whenever it is learned even for addresses
for which the corresponding value of gDot1qTpFdbStatus is
not learned(3).