• Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on the port, or no spanning-tree instance running on the port. An interface moves through these states: • From initialization to blocking • From blocking to listening or to disabled • From listening to learning or to disabled • From learning to forwarding or to disabled • From forwarding to disabled Figure 1: Spanning-Tree Interface States An interface moves through the states. When you power up the device, spanning tree is enabled by default, and every interface in the device, VLAN, or network goes through the blocking state and the transitory states of listening and learning.
Spanning tree stabilizes each interface at the forwarding or blocking state. When the spanning-tree algorithm places a Layer 2 interface in the forwarding state, this process occurs: 1. The interface is in the listening state while spanning tree waits for protocol information to move the interface to the blocking state. 2. While spanning tree waits for the forward-delay timer to expire, it moves the interface to the learning state and resets the forward-delay timer. 3. In the learning state, the interface continues to block frame forwarding as the device learns end-station location information for the forwarding database. 4.
When the forward-delay timer expires, spanning tree moves the interface to the forwarding state, where both learning and frame forwarding are enabled. Blocking State A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU is sent to each device interface. A device initially functions as the root until it exchanges BPDUs with other devices. This exchange establishes which device in the network is the root or root device. If there is only one device in the network, no exchange occurs, the forward-delay timer expires, and the interface moves to the listening state. An interface always enters the blocking state after device initialization.
Configuring Spanning Tree Protocol Blocking State Configuring Spanning Tree Protocol 5