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How Spanning Tree Protocols Work
•
Blocking
•
Listening
•
Learning
•
Forwarding
Disabled
•
A port moves through these states:
From initialization to blocking
•
From blocking to either listening or disabled
•
•
From listening to either listening or disabled
•
From learning to either forwarding or disabled
•
From forwarding to disabled
Figure 7-2
Figure 7-2
Boot-up
initialization
Blocking
state
Listening
state
Learning
state
Forwarding
state
You can modify each port state by using management software, such as VLAN Trunk Protocol (VTP).
When you enable spanning tree, every switch in the network goes through the blocking state and the
transitory states of listening and learning at power up. If properly configured, each port stabilizes into
the forwarding or blocking state.
When the spanning tree algorithm places a port in the forwarding state, the following occurs:
•
The port is put into the listening state while it waits for protocol information that suggests it should
go to the blocking state.
•
The port waits for the expiration of a protocol timer that moves the port to the learning state.
•
In the learning state, the port continues to block frame forwarding as it learns station location
information for the forwarding database.
•
The expiration of a protocol timer moves the port to the forwarding state, where both learning and
forwarding are enabled.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
7-6
illustrates how a port moves through the states.
STP Port States
Disabled
state
Chapter 7
Configuring Spanning Tree
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