Spanning Tree Port States

Spanning tree protocol prevent looping of frames around LAN by placing ports of switch in different states based on the criteria such as selection of root switch, switch’s root cost, etc.

There are five Spanning Tree Port States :

1. Blocking State :

• Switch port enters the blocking state at time of election process, when a switch receives a BPDU on a port that indicates a better path to the Root Switch or if a port is not a Root Port.
• Port discards the frames received from attached network segments or switched from another port for forwarding.
• Port receives BPDUs and directs them to the switch system module for processing.
• Port receives and responds to network management messages.
• Switch port remains in blocking state for 20 seconds and later changes to listening state.

2. Listening State :

• Listening state is the first transitional state after blocking state.
• In this state, STP determines whether the port should participate in frame forwarding or not.
• Port discards frames received from the attached network segment or switched from another port.
• Port receives BPDUs and directs them to the switch system module for processing. Also, it process BPDUs received from the switch system module.
• Port receives and responds to network management messages.
• Switch port remains in listening state for 15 seconds and then move to learning state.

3. Learning State :

• Switch port enters learning state after passing through the listening state as per the decision taken by STP.
• Port discards frames received from the attached network segment or switched from another port for forwarding.
• Port incorporates LAN host location information into its MAC address database.
• Port receive, process, and transmit BPDUs received from the system module.
• Port receives and responds to network management messages.
• Switch port remains in learning state for 15 seconds and then move to forwarding state.

4. Forwarding State :

• Switch port enters forwarding state after passing through all the states involved in creation of network topology.
• Port forwards the frames received from the attached network segment or switched from another port.
• The port is forwarding traffic.
• Port incorporates LAN host location information into the MAC address database.
• Port receive, process, and transmit BPDUs received from the system module.
• Port receives and responds to network management messages.
• Switch port remains in forwarding state until any change is detected in network.

5. Disabled State :

• Switch port enters the disabled state as a result of an administrative command to disable the port.
• Port does not participate in any operation of STP because it is considered non-operational.
• Port does not direct received BPDUs to the switch system module for processing.
• Port receives and responds to network management messages.

STP Modes	Receive BPDUs	Send BPDUs	Learn MAC Address	Forward Data Packets
Blocking	Yes	Yes	No	No
Listening	Yes	Yes	No	No
Learning	Yes	Yes	Yes	No
Forwarding	Yes	Yes	Yes	Yes
Disabled	Yes	No	No	No

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