Spanning Tree Port States, Blocking, Listening, Learning, Forwarding, Disabled

The ports on a switch with enabled Spanning Tree Protocol (STP) are in one of the following five port states.

• Blocking

• Listening

• Learning

• Forwarding

• Disabled

A switch does not enter any of these port states immediately except the blocking state. When the Spanning Tree Protocol (STP) is enabled, every switch in the network starts in the blocking state and later changes to the listening and learning states.

Blocking State

The Switch Ports will go into a blocking state at the time of election process, when a switch receives a BPDU on a port that indicates a better path to the Root Switch (Root Bridge), and if a port is not a Root Port or a Designated Port.

A port in the blocking state does not participate in frame forwarding and also discards frames received from the attached network segment. During blocking state, the port is only listening to and processing BPDUs on its interfaces. After 20 seconds, the switch port changes from the blocking state to the listening state.

Listening State

After blocking state, a Root Port or a Designated Port will move to a listening state. All other ports will remain in a blocked state. During the listening state the port discards frames received from the attached network segment and it also discards frames switched from another port for forwarding. At this state, the port receives BPDUs from the network segment and directs them to the switch system module for processing. After 15 seconds, the switch port moves from the listening state to the learning state.

Learning State

A port changes to learning state after listening state. During the learning state, the port is listening for and processing BPDUs . In the listening state, the port begins to process user frames and start updating the MAC address table. But the user frames are not forwarded to the destination. After 15 seconds, the switch port moves from the learning state to the forwarding state.

Forwarding State

A port in the forwarding state forwards frames across the attached network segment. In a forwarding state, the port will process BPDUs , update its MAC Address table with frames that it receives, and forward user traffic through the port. Forwarding State is the normal state. Data and configuration messages are passed through the port, when it is in forwarding state.

Disabled State

A port in the disabled state does not participate in frame forwarding or the operation of STP because a port in the disabled state is considered non-operational.