A packet switch appliance includes a plurality of ports. One of the plurality of ports is configured to operate as a network port connected to a packet-switching network. Another of the plurality of ports is configured to operate as a first instrument port connected to a network instrument. To filter packets, one or more packets or copies of packets received through the first network port are examined prior to the packets or copies of packets being sent out the first instrument port to determine a current state of a state-based protocol, which includes a plurality of potential states. A filter is created or modified for the first network port or the first instrument port based on the determined current state of the state-based protocol.