The STP (spanning-Tree Protocol) runs on bridges and switches exchange information through the BPDUs (Bridge Protocol Data Units). These BPDUs contain configuration messages and include the ID of each bridge and are sent via broadcast frames.
For operation of an STP Root Bridge is elected (the root switch). This switch-root becomes the focus of the network, and the choices of the root port, port-designated and non-designated network and all updates are seen by the prospect of the root switch. To elect him the following parameters are used:

1st Lower priority value (default is 32768);
2nd Minor MAC Address (0000.aaaa.bbbb.cccc.dddd.eeee is less than 0000.aaa b. Bbbb.cccc.dddd.eeee).

The doors may have the following roles:

Role	Description
Root port	port on a switch that has lower cost to root bridge. Where are the forwarding state).
Designated port	port on a switch to maintain connectivity. When there are redundant connections, the port with higher bandwidth is chosen. If the band widths are the same, the port with the lowest number will be chosen (eg e1 chosen instead of E8).
Non-designated port	port that is in blocking state. When a designated port loses connectivity to this port is, until it receives back to update the state blocking

The figure below shows a topology with the terms discussed above:

Have seen the papers that have doors. Now, will see the states where they can find. Typically, the switches are blocking orforwarding state. Below is presented each of the states in the sequence in which they occur.

State	Description
Blocking	Do not forward frames. Receives and analyzes BPDUs. By connecting a switch, it is in blocking state.
Listening	Do not forward frames, but receives and analyzes BPDUs to make sure that no loops occur on the network.
Learning	Do not forward frames, and receives BPDUs analyzes and records the MAC addresses of devices connected directly.
Forwarding	Sends and receives frames and everything. A port in this state is regarded as having the lowest cost to the root switch.

Below is presented the timers of BPDUs:

Below is a flash animation that presents very intuitive the process of learning the spanning-Tree. A MUST.

Below, follow the links to reference that were used to produce the summary.

An interesting, which deepens the matter is the third reference. Can be found translated (by Google) here.

A Abrasive,

Maurício.

References:

- Filippeti, M. - “CCNA 4.1 - Complete Study Guide, Visual Books/2008.

- Cisco NetAcademy, Academy Connection. “Cisco Study Material official.” <http://cisco.netacad.net>. Acesso em: 03/03/2008.

- Cisco Systems inc. “Understanding and Configuring spanning Tree Protocol (STP) on Catalyst Switches”.<Http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml>. Acesso em: 03/03/2008.

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