Content

| 3.2 |  | STP Processes |
| 3.2.6 |  | STP states |

STP States
After the bridges have determined which ports are Root Ports, Designated Ports, and non-Designated Ports, STP is ready to create a loop-free topology. To do this, STP configures Root Ports and Designated Ports to forward traffic. STP sets non-Designated Ports to block traffic. Although Forwarding and Blocking are the only two states commonly seen in a stable network, there are actually five STP states.

This list can be viewed hierarchically in that bridge ports start at the Blocking state and work their way up to the Forwarding state. The Disabled state is the administratively shutdown STP state. It is not part of the normal STP port processing. After the switch is initialized, ports start in the Blocking state. The Blocking state is the STP state in which a bridge listens for BPDUs.

A port in the Blocking state does the following:

- Discards frames received from the attached segment or internally forwarded through switching
- Receives BPDUs and directs them to the system module
- Has no address database
- Does not transmit BPDUs received from the system module
- Receives and responds to network management messages but does not transmit them

If a bridge thinks it is the Root Bridge immediately after booting or in the absence of BPDUs for a certain period of time, the port transitions into the Listening state. The Listening state is the STP state in which no user data is being passed, but the port is sending and receiving BPDUs in an effort to determine the active topology.

A port in the Listening state does the following:

- Discards frames received from the attached segment or frames switched from another port
- Has no address database
- Receives BPDUs and directs them to the system module
- Processes BPDUs received from the system module (Processing BPDUs is a separate action from receiving or transmitting BPDUs)
- Receives and responds to network management messages

It is during the Listening state that the three initial convergence steps take place elect a Root Bridge, elect Root Ports, and elect Designated Ports. Ports that lose the Designated Port election become non-Designated Ports and drop back to the Blocking state. Ports that remain Designated Ports or Root Ports after 15 seconds the default Forward Delay STP timer value progress into the Learning state. The lifetime of the Learning state is also governed by the Forward Delay timer of 15 seconds, the default setting.

The Learning state is the STP state in which the bridge is not passing user data frames but is building the bridging table and gathering information, such as the source VLANs of data frames. As the bridge receives a frame, it places the source MAC address and port into the bridging table. The Learning state reduces the amount of flooding required when data forwarding begins.

A port in the Learning state does the following:

- Discards frames received from the attached segment
- Discards frames switched from another port for forwarding
- Incorporates station location into its address database
- Receives BPDUs and directs them to the system module
- Receives, processes, and transmits BPDUs received from the system module
- Receives and responds to network management messages

If a port is still a Designated Port or Root Port after the Forward Delay timer expires for the Learning state, the port transitions into the Forwarding state. The Forwarding state is the STP state in which data traffic is both sent and received on a port. It is the last STP state. At this stage, it finally starts forwarding user data frames.

A port in the Forwarding state does the following:

- Forwards frames received from the attached segment
- Forwards frames switched from another port for forwarding
- Incorporates station location information into its address database
- Receives BPDUs and directs them to the system module
- Processes BPDUs received from the system module
- Receives and responds to network management messages