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Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 13 Configuring STP
Understanding Spanning-Tree Features
Figure 13-1
shows how an interface moves through the states.
Figure 13-1
Spanning-Tree Interface States
When you power up the switch, spanning tree is enabled by default, and every port on the switch, as well
as any other port in other switches in the VLAN or network that are participating in spanning tree, 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 spanning-tree interface in the forwarding state, this
process occurs:
1.
The interface is in the listening state while spanning tree waits for protocol information to transition
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.
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, or to each switch STP port. 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 participating in
spanning tree 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
Power-on
initialization
Blocking
state
43569
Listening
state
Disabled
state
Learning
state
Forwarding
state
