Either the value '0', or the port on which a
frame having a source address equal to the value
of the corresponding instance of dot1dTpFdbAddress
has been seen. A non-zero value of this object
indicates the port for which the ifIndex object,
defined in [4,6], of the corresponding interface
has the same value. 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).