This attribute specifies the name of a protocol port whereby networks
learned over this port are to be filtered or passed depending on the
disposition of the filter. This attribute is only valid for filters applied to a
port in the output direction and will be ignored if the filter is applied in
the input direction. This attribute is entered as an ascii string and must
correspond to a name of a provisioned protocol port. A port name
preceded by a caret (^) should be interpreted as matching all RIP packet
tuples except for those learned on this port. For a given filter, only
networks matching all the specified attributes of that filter are filtered or
passed depending on the disposition of the filter.