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STP Terms and Concepts 181
Figure 7 illustrates the factors that cause a port to change from one state
to another. The arrows indicate the direction of movement between
states. The numbers correspond to the factors that affect the transition.
Figure 7 Factors Involved in Spanning Tree Port State Transitions
As shown in Figure 7, for a port in the blocking state to transition to the
listening state, STP must select that port as a designated or root port.
After the port enters the listening state, the forward delay must expire
before the port can transition to the learning state. Then another forward
delay must expire before the port can transition to the forwarding state. If
you disable a port in the listening, learning, or forwarding state or if port
initialization fails, then that port becomes disabled.
Disabled Blocking Listening Learning Forwarding
Port enabled by either network administrator or initialization
Port disabled by either network administrator or failure
Spanning Tree algorithm selects port as designated or root
Spanning Tree algorithm does not select port as designated or root
Forwarding timer (forward delay) expires
and 1 2
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