Specifications

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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 12 Configuring STP
Understanding Spanning-Tree Features
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 switch 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 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.
An 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 interface enters
this state when the spanning tree determines that the interface should participate in frame forwarding.
An 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 interface enters
the learning state from the listening state.
An interface in the learning state performs as follows:
• Discards frames received on the port
• Discards frames switched from another interface for forwarding
• Learns addresses
• Receives BPDUs