Each Layer 2 interface participating in a spanning tree is in one of five states.
A network topology of bridges typically contains redundant connections to provide alternate paths in case of link failures. The redundant connections create a potential for loops in the system. As there is no concept of time to live (TTL) in Ethernet frames, a situation may arise where there is a permanent circulation of frames when the network contains loops. To prevent this, a spanning tree connecting all the bridges is formed in real time.
| State | Action or inaction |
|---|---|
| Blocking | The interface does not forward frames. Redundant ports are put in a blocking state and enabled when required. This is a transitional state after initialization. |
| Listening | The interface is identified by the spanning tree as one that should participate in frame forwarding. This is a transitional state after the blocking state for a legacy STP. |
| Learning | The interface prepares to participate in frame forwarding. This is a transitional state after the blocking state for a legacy STP. |
| Forwarding | The interface forwards frames. This is a transitional state after the learning state. |
| Disabled | The interface is not participating in a spanning tree because of shutdown of a port or the port is not operationally up. Any of the other states may transition into this state. |