Section 4.4: STP States
To participate in STP, each port of a switch must progress through several states. A port begins in a
Disabled state moving through several passive states and finally into an active state if allowed to forward
traffic. The STP port states are: Disabled, Blocking, Listening, Learning, and Forwarding.
• Ports that are administratively shut down by the network administrator or by the system due to a fault condition are in the Disabled state. This state is special and is not part of the normal STP progression for a port.
• After a port initializes, it begins in the Blocking state so that no bridging loops can form. In the Blocking state, a port cannot receive or transmit data and cannot add MAC addresses to its address table. Instead, a port is only allowed to receive BPDUs. Also, ports that are put into standby mode to remove a bridging loop enter the Blocking state.
• The port will be moved from the Blocking state to the Listening state if the switch thinks that the port can be selected as a Root Port or Designated Port. In the Listening state, the port still cannot send or receive data frames. However, the port is allowed to receive and send BPDUs so that it can actively participate in the Spanning-Tree topology process. Here the port is finally allowed to become a Root Port or Designated Port because the switch can advertise the port by sending BPDUs to other switches. Should the port lose its Root Port or Designated Port status, it is returned to the Blocking state.
• After a period of time called the Forward Delay in the Listening state, the port is allowed to move into the Learning state. The port still sends and receives BPDUs as before. In addition, the switch can now learn new MAC addresses to add into its address table.
• After another Forward Delay period in the Learning state, the port is allowed to move into the Forwarding state. The port can now send and receive data frames, collect MAC addresses into its address table, and send and receive BPDUs. The port is now a fully functioning switch port within the Spanning-Tree topology.