cisco Rapid PVST+ Port State

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Port States

Rapid PVST+ Port State Overview
Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When a LAN port transitions directly from nonparticipation in the spanning tree topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switched LAN before starting to forward frames.

Each LAN port on a software using Rapid PVST+ or MST exists in one of the following four states:

Blocking—The LAN port does not participate in frame forwarding.
Learning—The LAN port prepares to participate in frame forwarding.
Forwarding—The LAN port forwards frames.
Disabled—The LAN port does not participate in STP and is not forwarding frames.
When you enable Rapid PVST+, every port in the software, VLAN, and network goes through the blocking state and the transitory states of learning at power up. If properly configured, each LAN port stabilizes to the forwarding or blocking state.

When the STP algorithm places a LAN port in the forwarding state, the following process occurs:

The LAN port is put into the blocking state while it waits for protocol information that suggests it should go to the learning state.
The LAN port waits for the forward delay timer to expire, moves the LAN port to the learning state, and restarts the forward delay timer.
In the learning state, the LAN port continues to block frame forwarding as it learns the end station location information for the forwarding database.
The LAN port waits for the forward delay timer to expire and then moves the LAN port to the forwarding state, where both learning and frame forwarding are enabled.
Blocking State
A LAN port in the blocking state does not participate in frame forwarding.

A LAN port in the blocking state performs as follows:

Discards frames received from the attached segment.
Discards frames switched from another port for forwarding.
Does not incorporate the end station location into its address database. (There is no learning on a blocking LAN port, so there is no address database update.)
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.
Learning State
A LAN port in the learning state prepares to participate in frame forwarding by learning the MAC addresses for the frames. The LAN port enters the learning state from the blocking state.

A LAN port in the learning state performs as follows:

Discards frames received from the attached segment.
Discards frames switched from another port for forwarding.
Incorporates the end 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.
Forwarding State
A LAN port in the forwarding state forwards frames. The LAN port enters the forwarding state from the learning state.

A LAN port in the forwarding state performs as follows:

Forwards frames received from the attached segment.
Forwards frames switched from another port for forwarding.
Incorporates the end 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.
Disabled State
A LAN port in the disabled state does not participate in frame forwarding or STP. A LAN port in the disabled state is virtually nonoperational.

A disabled LAN port performs as follows:

Discards frames received from the attached segment.
Discards frames switched from another port for forwarding.
Does not incorporate the end station location into its address database. (There is no learning, so there is no address database update.)
Does not receive BPDUs from neighbors.
Does not receive BPDUs for transmission from the system module.

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