Spanning-Tree Port States

The ports on a bridge or switch running the STP can transition through four

different states:
 Won’t forward frames; listens to BPDUs. All ports are in
blocking state by default when the switch is powered up.
Listens to BPDUs to make sure no loops occur on the network
before passing data frames.

Learns MAC addresses and builds a filter table but does not
forward frames.
Sends and receives all data on the bridged port.
Typically, switch ports are in either blocking or forwarding state.
A forwarding
port has been determined to have the lowest cost to the root bridge.
However, if the network has a topology change because of a failed link or
even if the administrator adds a new switch to the network, the ports on a
switch will be in listening and learning state.
Blocking ports are used to prevent network loops. Once a switch determines
the best path to the root bridge, then all other ports will be in blocking
state. Blocked ports still receive BPDUs.
If a switch determines that a blocked port should now be the designated
port, it will go to listening state. It will check all BPDUs heard to make sure
that it won’t create a loop once the port goes to forwarding state.
Disabled State
A port in the disabled state does not participate in frame forwarding or the operation of STP because a port in the disabled state is considered non-operational.

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