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Understanding Spanning Tree Protocol
Figure 2
Figure 2
Power-on
initialization
Forwarding
When you enable STP on the 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.
2.
3.
4.
Blocking State
An interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU
is sent to the bridge's Ethernet and radio ports. An bridge initially functions as the spanning-tree root
until it exchanges BPDUs with other bridges. This exchange establishes which bridge in the network is
the spanning-tree root. If there is only one 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:
•
•
•
Book Title
6
illustrates how an interface moves through the states.
Spanning-Tree Interface States
Blocking
state
Listening
Disabled
state
state
Learning
state
state
The interface is in the listening state while spanning tree waits for protocol information to transition
the interface to the blocking state.
While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning
state and resets the forward-delay timer.
In the learning state, the interface continues to block frame forwarding as the bridge learns
end-station location information for the forwarding database.
When the forward-delay timer expires, spanning tree moves the interface to the forwarding state,
where both learning and frame forwarding are enabled.
Discards frames received on the port
Does not learn addresses
Receives BPDUs
Spanning Tree Protocol in a Wireless Environment
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