Chapter 14
Configuring STP
•
Forwarding—The interface forwards frames.
•
Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on
the port, or no spanning-tree instance running on the port.
An interface moves through these states:
•
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 14-1
Figure 14-1 Spanning-Tree Interface States
Power-on
initialization
Blocking
state
Listening
state
Learning
state
Forwarding
state
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:
The interface is in the listening state while spanning tree waits for protocol information to transition
1.
the interface to the blocking state.
While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning
2.
state and resets the forward-delay timer.
In the learning state, the interface continues to block frame forwarding as the switch learns
3.
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.
78-11380-10
illustrates how an interface moves through the states.
Disabled
state
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
Understanding Spanning-Tree Features
14-5