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 dot1dTpFdbAddress 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 (e.g. in the dot1dStaticTable).
Implementors are encouraged to assign the port
value to this object whenever it is learned even
for addresses for which the corresponding value of
dot1dTpFdbStatus is not learned(3).