Each port on a switch using STP exists in one of the following five states:
Blocking
The port is blocked from forwarding or receiving packets. For additional
information, see “Blocking state” on page 127.
Listening
The port is waiting to receive BPDU packets that might tell the port to go
back to the blocking state. For additional information, see “Listening state”
on page 128.
Learning
The port is adding addresses to its forwarding database but not yet
forwarding packets. For additional information, see “Learning state” on
page 129.
Forwarding
The port is forwarding packets. For additional information, see “Forwarding
state” on page 130.
Disabled
The port responds only to network management messages and must return
to the blocking state first. For additional information, see “Disabled state” on
page 132.
A port changes from one state to another as follows:
v From initialization (switch startup) to blocking
v From blocking to listening or to disabled
v From listening to learning or to disabled
v From learning to forwarding or to disabled
v From forwarding to disabled
v From disabled to blocking
Appendix D. Understanding and troubleshooting the Spanning Tree Protocol 123
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