The switch is expected to send port-status messages to the controller as the port configuration state changes. This is done using the OFPT_PORT_STATUS message. The description field of the message signifies the action that has taken place by the port, 0 being ADD, 1 being DELETE, and 2 signifying some attributes of the port that has changed. An example of such events will include the change in port status (for example, disabled by the user) or a change in the port status, as specified by 802.1D (spanning tree). OpenFlow switches may optionally support 802.1D Spanning Tree Protocol (STP).These switches are expected to process all 802.1D packets locally before performing flow lookup. Ports status, as specified by the STP, is then used to limit packets forwarded to the OFP_FLOOD port to only those ports along the spanning tree. Port changes as a result of the spanning tree are sent to the controller via the port-update messages. Note that forward actions that specify the outgoing port of OFP_ALL ignore the port status set by the STP. Packets received on the ports that are disabled by the STP must follow the normal flow table processing path.