Cisco Systems 3750E manual 2illustrates how an interface moves through the states, 18-6

Chapter 18 Configuring STP

Understanding Spanning-Tree Features

•From learning to forwarding or to disabled

•From forwarding to disabled

Figure 18-2illustrates how an interface moves through the states.

Figure 18-2

Power-on

initialization

Blocking

state

Listening

state

Learning

state

Forwarding

state

Spanning-Tree Interface States

Disabled

state

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When you power up the switch, spanning tree is enabled by default, and every interface in the switch, VLAN, or network goes 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.The interface is in the listening state while spanning tree waits for protocol information to move 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.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

18-6 OL-9775-02