Spanning Tree Port States

Spanning Tree Port States
The ports on a bridge or switch running the STP will go through four transitional states:
Blocking Won’t forward frames; listens to BPDUs. All ports are in blocking state by default
when the switch is powered on.
Listening Listens to BPDUs to make sure no loops occur on the network before passing
data frames.
Learning Learns MAC addresses and builds a filter table, but does not forward frames.
Forwarding Bridge port is able to send and receive data. A port is never placed in forwarding
state unless there are no redundant links or the port determines that it has the best path to the
root bridge.
An administrator can put a port in disabled state, or if a failure with the port occurs, the
switch puts it into disabled state.
Typically, switch ports are in either blocking or forwarding state. A forwarding port is a port
that 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 the administrator adds a new switch to the network,
the ports on a switch will be in listening and learning states.
Blocking ports are used to prevent network loops. After a switch determines the best path to
the root bridge, all other ports may be placed in the blocking state. Blocked ports will still
receive BPDUs.
If a switch determines that a blocked port should now be the designated port, it will go to listening
state. It checks all BPDUs heard to make sure that it won’t create a loop after the port
goes to forwarding state.
Figure 15.5 shows the default STP timers and their operation within STP.