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Cisco IOS Software Configuration Guide for Cisco Aironet Access Points
OL-30644-01
Chapter 8 Configuring Spanning Tree Protocol
Understanding Spanning Tree Protocol
• 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 8-2 illustrates how an interface moves through the states.
Figure 8-2 Spanning-Tree Interface States
When you enable STP on the access point/bridge, the Ethernet and radio interfaces go 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 transition
the interface to the blocking state.
2. While spanning tree waits 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 access point/bridge
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
An interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU
is sent to the access point/bridge’s Ethernet and radio ports. A access point/bridge initially functions as
the spanning-tree root until it exchanges BPDUs with other access point/bridges. This exchange
establishes which access point/bridge in the network is the spanning-tree root. If there is only one access
point/bridge in the network, no exchange occurs, the forward-delay timer expires, and the interfaces
move to the listening state. An interface always enters the blocking state when you enable STP.
An interface in the blocking state performs as follows:
Power-on
initialization
Blocking
state
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Listening
state
Disabled
state
Learning
state
Forwarding
state