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Chapter 16
Configuring STP
An interface moves through these states:
•
•
•
•
•
Figure 16-1
Figure 16-1
Power-on
initialization
Blocking
Listening
Learning
Forwarding
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.
2.
3.
4.
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
OL-8915-03
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
illustrates how an interface moves through the states.
Spanning-Tree Interface States
state
Disabled
state
state
state
state
The interface is in the listening state while spanning tree waits for protocol information to move the
interface to the blocking state.
While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning
state and resets the forward-delay timer.
In the learning state, the interface continues to block frame forwarding as the switch learns
end-station location information for the forwarding database.
When the forward-delay timer expires, spanning tree moves the interface to the forwarding state,
where both learning and frame forwarding are enabled.
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
Understanding Spanning-Tree Features
16-5
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