If properly con?¬?gured, the ports then
stabilize to the forwarding or blocking state. Forwarding ports provide the lowest-cost path
to the root bridge. During a topology change, a port temporarily implements the listening
and learning states.
The disabled state is not strictly part of STP; a network administrator can manually disable
a port, or a security or an error condition may disable it. An example of a port that is
disabled would be a port that is shut down.
Figure 2-24 shows the ?¬‚ow of spanning-tree port states.
Figure 2-24 Spanning-Tree Port States
All bridge ports initially start in the blocking state, from which they listen for BPDUs.
When the bridge ?¬?rst boots, it functions as if it were the root bridge and transitions to the
listening state. An absence of BPDUs for a certain period is called the maximum age
(max_age), which has a default of 20 seconds. If a port is in the blocking state and does not
receive a new BPDU within the max_age, the bridge transitions from the blocking state to
the listening state. When a port is in the transitional listening state, it can send and receive
BPDUs to determine the active topology. At this point, the switch is not passing user data.
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