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Configuring STP
Information About Configuring STP
Learning—The interface prepares to participate in frame forwarding.
Forwarding—The interface forwards frames.
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 36 on page 318 illustrates how an interface moves through the states.
Figure 36 Spanning-Tree Interface States
When you power up the switch, spanning tree is enabled by default, and every interface in the switch, 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 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 switch learns end-station location
information for the forwarding database.
Power-on
initialization
Blocking
state
43569
Listening
state
Disabled
state
Learning
state
Forwarding
state
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