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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
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Chapter 8 Configuring Spanning Tree
Understanding How Spanning Tree Protocols Work
At any given time, each port on a switch using a spanning tree protocol is in one of these states:
•
Blocking
•
Listening
•
Learning
•
Forwarding
•
Disabled
A port moves through these states as follows:
•
From initialization to blocking
•
From blocking to listening or to disabled
•
From listening to learning or to disabled
•
From learning to forwarding or to disabled
•
From forwarding to disabled
Figure 8-2
illustrates how a port moves through the states.
Figure 8-2
STP Port States
You can modify each port state by using management software, for example, VLAN Trunking 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.
Boot-up
initialization
Blocking
state
S5691
Listening
state
Disabled
state
Learning
state
Forwarding
state
