18-7
Catalyst 3750 Switch Software Configuration Guide
OL-8550-02
Chapter 18 Configuring STP
Understanding Spanning-Tree Features
Blocking State
A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a
BPDU is sent to each switch interface. 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
interface moves to the listening state. An interface always enters the blocking state after switch
initialization.
An interface in the blocking state performs these functions:
• Discards frames received on the interface
• 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 decides that the interface should participate in frame forwarding.
An interface in the listening state performs these functions:
• Discards frames received on the interface
• 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 these functions:
• Discards frames received on the interface
• 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 these functions:
• Receives and forwards frames received on the interface
• Forwards frames switched from another interface
• Learns addresses
• Receives BPDUs
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