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Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter 16 Configuring STP
Understanding Spanning-Tree Features
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
Forwarding State
A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state
from the learning state.
An interface in the forwarding state performs as follows:
• Receives and forwards frames received on the port
• Forwards frames switched from another port
• Learns addresses
• Receives BPDUs
Disabled State
A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree.
An interface in the disabled state is nonoperational.
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