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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 11 Configuring STP
Unders tanding Spanni ng-Tree Fea tures
An interface moves through these states:
•From initialization to blocking
•From bl ocki ng to li ste ning or to disab l ed
•From list eni ng to le arni ng o r t o di sabl ed
•From le arning t o fo rwardi ng or t o disabl ed
•From for war ding to d i sable d
Figure 11-2 illustrates how an interface moves through the states.
Figure 11-2 Spanning-Tree Interface States
When you power up the switch, STP is enabled by default, and every inte rface in the switch, VLAN, or
network goes through the blocking state and the transitory states of listening and learning. Spanning tree
stabili zes each interfac e at the forwa rding or bl ocking stat e.
When th e s pannin g-tree al gorith m pla ces a Lay er 2 i nte rf ace i n the forw ar ding state , thi s p roces s occu rs:
1. The interf ace is in the liste ning state while spanning tree wai ts for protocol infor mation 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-s tation l ocation i nform ation for the forwa rding data base.
4. When the forward-delay timer expires, spanning tree moves the interface to the forwarding state,
where both learning and frame forwarding are enabled.
Power-on
initialization
Blocking
state
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Listening
state Disabled
state
Learning
state
Forwarding
state