The IEEE 802.1D Spanning Tree Protocol (STP) runs on bridges and switches that are
802.1D-compliant. STP prevents loops in the network by providing redundant links. If a primary link
fails, the backup link is activated and network traffic is not affected. Without STP running on the
switch or bridge, a link failure can result in a loop.

When the spanning tree algorithm is run, the network switches transform the real network topology
into a spanning tree topology in which any LAN in the network can be reached from any other LAN
through a unique path. The network switches recalculate a new spanning tree topology whenever
there is a change to the network topology.

For each LAN, the switches that attach to the LAN choose a designated switch that is the closest
switch to the root switch. This designated switch is responsible for forwarding all traffic to and from
the LAN. The port on the designated switch that connects to the LAN is called the designated port.

The switches decide which of their ports will be part of the spanning tree. A port is included in the
spanning tree if it is a root port or a designated port.

With STP, data traffic is allowed only on those ports that are part of the spanning tree topology.
Ports that are not part of the spanning tree topology are automatically changed to a blocking
(inactive) state. They are kept in the blocking state until there is a break in the spanning tree
topology, at which time they are automatically activated to provide a new path.

The STP interface states for every Layer 2 interface running STP are as follows:

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Blocking—The interface does not forward frames.

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Listening—The interface is identified by the spanning tree as one that should participate in
frame forwarding. This is a transitional state after the blocking state.

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Learning—The interface prepares to participate in frame forwarding.

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Forwarding—The interface forwards frames.