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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 10 Configuring STP
Understanding Basic STP Features
When the spanning-tree algorithm places a Layer 2 interface in the forwarding state, this process occurs:
1.
The Layer 2 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 Layer 2 interface to the
learning state and resets the forward-delay timer.
3.
In the learning state, the Layer 2 interface continues to block frame forwarding as the switch learns
end-station location information for the forwarding database.
4.
When the forward-delay timer expires, spanning tree moves the Layer 2 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 Layer 2 interface in the switch. A switch initially functions as the root until it
exchanges BPDUs with other switches. This exchange establishes which switch in the network is the
root or root switch. If there is only one switch 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
after switch initialization.
A Layer 2 interface in the blocking state performs as follows:
•
Discards frames received on the port
•
Discards frames switched from another interface for forwarding
•
Does not learn addresses
•
Receives BPDUs
Listening State
The listening state is the first state a Layer 2 interface enters after the blocking state. The Layer 2
interface enters this state when STP determines that the Layer 2 interface should participate in frame
forwarding.
A Layer 2 interface in the listening state performs as follows:
•
Discards frames received on the port
•
Discards frames switched from another interface for forwarding
•
Does not learn addresses
•
Receives BPDUs
Learning State
A Layer 2 interface in the learning state prepares to participate in frame forwarding. The Layer 2
interface enters the learning state from the listening state.
A Layer 2 interface in the learning state performs as follows:
•
Discards frames received on the port
•
Discards frames switched from another interface for forwarding
